Watson's Biblical & Theological Dictionary 
a comprehensive term, including all who believe the Deity to subsist in one person only. The chief article in the religious system of the Unitarians is, that Christ was a mere man. But they consider him as the great instrument in the hands of God of reversing all the effects of the fall; as the object of all the prophecies from Moses to his own time; as the great bond of union to virtuous and good men, who, as Christians, make one body in a peculiar sense. The Socinian creed was reduced to what Dr. Priestly calls Humanitarianism, by denying the miraculous conception, the infallibility, and the impeccability of the Saviour; and, consequently, his right to any divine honours or religious worship. As to those texts which declare that Jesus Christ "knew no sin," &c, his followers explain them in the sense in which it is said of believers, "Whosoever is born of God doth not commit sin," 1 John 3:9 . Or, if this be not satisfactory, Dr. Priestly refers us to the "Theological Repository," "in which," he says, "I think I have shown that the Apostle Paul often reasons inconclusively; and, therefore, that he wrote as any other person of his turn of mind or thinking, and in his situation, would have written, without any particular inspiration. Facts, such as I think I have there alleged, are stubborn things, and all hypotheses must be accommodated to them." Nor is this sentiment peculiar to Dr. Priestley. Mr. Belsham says, "The Unitarian doctrine is, that Jesus of Nazareth was a man constituted in all respects like other men, subject to the same infirmities, the same ignorance, prejudices, and frailties; descended from the family of David, the son of Joseph and Mary, though some indeed still adhere to the popular opinion of the miraculous conception; that he was born in low circumstances, having no peculiar advantages of education or learning, but that he was a man of exemplary character; and that, in conformity to ancient prophecy, he was chosen and appointed by God to introduce a new moral dispensation into the world, the design of which was to abolish the Jewish economy, and to place believing Gentiles upon an equal ground of privilege and favour with the posterity of Abraham; in other words, he was authorized to reveal to all mankind, without distinction, the great doctrine of a future life, in which men shall be rewarded according to their works." Mr. Belsham goes on to state the Unitarian opinion to be, that Jesus was not conscious of his high character till after his baptism; that he afterward spent some time in the wilderness, where he was invested with miraculous powers, and favoured with heavenly visions, like St. Paul, 2 Corinthians 12, in which he supposed himself taken up into heaven, and in consequence of which he speaks of his descent from heaven; that he exercised his ministry on earth for the space of a year or more, and then suffered death upon the cross, not to exhibit the evil of sin, or in any sense to make atonement for it, but as a martyr to the truth, and as a necessary preliminary to his resurrection, which they consider as a pledge of the resurrection of mankind. Many also believe that Jesus maintained some personal and sensible connection with the church during the apostolic age, and the continuance of miraculous powers in the church. They farther believe that he is appointed to revisit the earth, and to judge the world,—a difficult task one would suppose, if "he be constituted," as said above, "in all respects like other men, subject to the same ignorance, prejudices, frailties," &c! So this blasphemous system contains, in this respect, and in almost every other, its own refutation. See Socinians .
The creed which the celebrated council of Nice established, says Grier, in his "Epitome of General Councils," is that which Christians now profess; the errors and impieties which it condemned are those which, according to the refinements of Socinus, his followers of the present day have moulded into their antichristian system. Arius, a presbyter in the church of Alexandria, a man of consummate talent and address, but of a cold and speculative mind, impiously maintained that there had been a time when the Son of God was not; that he was capable of virtue and vice; and that he was a creature, and mutable as creatures are! It is true that Arius held a qualified preexistence, when he said that God created the Son from nothing before he created the world; in other words, that the Son was the first of created beings; but such preexistence does not imply coexistence or coeternity with the Father. After this manner did he deny the divinity of the Son, and his coeternity with the Father. Seduced by the pride of reasoning, no less than by his fondness for novelty, did he likewise reject the ομοουσιαν , as it is called, or the tenet of the Son being of the same substance with the Father. The blasphemies of Arius consisted in the denial of Christ's being either co-eternal or consubstantial with God. After a lapse of twelve centuries, Socinus lowered him another step by declaring his inferiority to the Father; for that he, as well as all other things, was subject to the supreme Creator of the universe; and although he held his mere humanity, yet, inconsistently enough, he would offer him divine worship! Inconsistently it may be said, because the Socinian, on his own principles, thereby incurs the guilt of idolatry as much as the Roman Catholic who worships the Virgin Mary, a mere created being. The Unitarian, or Humanitarian, sinks the character of the Saviour still lower, by withholding all worship from him; and while he considers him as a mere man, and therefore as not possessing the attributes of the Deity, with an inconsistency as singular as that of Socinus, he acknowledges his divinity so as to call him God; as if the terms Deity and Divinity bore different significations, or as if the principle which constituted the essence of the Godhead were separable from the Godhead itself! It should be observed, that the lowest denomination of unbelievers in the descending scale, namely, the modern Unitarian, combines with his own peculiar errors and impieties all the errors and impieties of both Arius and Socinus, together with an absolute denial of the Holy Ghost being a divine Person. Having touched on the shades of difference which exist between the followers of Arius and Socinus, a more minute detail of the division and subdivision of the classes into which they may be ranged may not be unacceptable to the reader: Arians and Semi-Arians constituted the original distraction; that of a subsequent day was high and low Arians. The high Arians entertain the highest views of the mediatorial influence of Christ, and believe in the entire Scriptures; the low Arians run into the opposite extreme, yet neither high nor low Arians consider Christ to be truly God. The old Socinians admitted the miraculous conception, and the worship of the Son; the modern Socinians do not; a circumstance that identifies the modern Socinian with the Unitarian. Some high Arians, such as Dr. Samuel Clarke, &c, thought that Christ might be worshipped; others of them affect to have no distinct notion of what the Holy Ghost meant, and to believe that worship is not to be addressed to Christ, but through Christ! These variations in the Unitarian creed have been deduced from the evidence of Unitarians themselves, given before the Commissioners of Education Inquiry in Ireland in 1826, as detailed in their Report to Parliament; a circumstance that renders them the more valuable, as it imparts to them a living, speaking authority. It must, however, be observed, that motley as they are, they all terminate in one point, the rejection of Christ's divinity; and that, diversified as the distinctions appear to be, they all will be ultimately found to be without a shadow of difference. In short, Arians, Socinians, Unitarians, &c, not only agree with each other in their antichristian scheme; but can scarcely be said to differ from the infidel Musselmans, who are taught by their Koran to regard Christ as a great prophet, and the forerunner of their own. With Deism doubtless Unitarianism has an intimate alliance. For Deists reject all the doctrines of the Christian revelation, while Unitarians reject all its peculiar doctrines:
2. The divinity of Christ.
3. The personality of the Holy Spirit.
4. The miraculous birth of Christ.
5. The atonement of Christ.
6. The sanctification of the Spirit.
7. The existence of angels and spirits;
8. And, therefore, of the devil and his angels.
"In what, then," says the learned Dr. Burgess, bishop of Salisbury, after this enumeration of the peculiar doctrines of Christianity, "does Unitarianism differ from Deism? Deists deny the essential doctrines of Christianity by rejecting the whole of the Christian revelation; Unitarians reject the Christian revelation by denying all its peculiar and essential doctrines."
Charles Buck Theological Dictionary 
Those who confine the glory and attributes of divinity to the Father, and not allowing it to the Son or Holy Spirit. They are the same as the Socinians.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
a general name for those bodies of professed Christians who do not fully recognize the equality of the three Persons in the Godhead. The essential errors of Unitarianism, as evangelical Trinitarians regard them, are a denial (a) of the true divinity of Jesus Christ; and (b) of the inherent and total moral depravity of human nature. These two are claimed to be not simply dogmas, but facts sustained by observation and history as well as by the plain and constant teachings of the Holy Scriptures. They are intimately correlated to each other; for if Christ be not truly divine, then there is no adequate atonement for human sin; and, conversely; if man be not essentially a sinner, he needs no such divine Savior. Hence our Lord in treating with Nicodemus announced the necessity of a radical, moral change as the first and all-important condition of Christianity (John 3, 1-13). Accordingly the doctrine of a spiritual and fundamental regeneration will be found to be the true touchstone of all evangelical orthodoxy, and those branches of Christendom who lay most stress upon it prove to be the most efficient in the moral renovation of mankind. Humanitarianism alone can never be more than a negative and powerless, because a really false, view of the actual condition and relation of the race as respects their Creator and Redeemer. (See Humanitarians).
In the same summary manner, Unitarians reject, as being to them unphilosophical and unintelligible, the divinity of the Holy Spirit, a doctrine which all who have passed through the pangs of true contrition into the joys of conscious pardon and heavenly communion find so comforting and necessary to the explanation of their own religious experience (Romans 5, 1-5; 1 Corinthians 2:10-14). (See Trinity).
While pointing out these, as we deem, radical defects in Unitarianism as a system of Christian faith, we nevertheless are bound to bear witness to the literary culture, social refinement, and moral virtues which Unitarians as a body have exhibited, and to their amenity and ameliorating influence in the defense of civil rights and the general cause of philanthropy. These we attribute, however, not so much to their creed as to the hereditary effect of early Puritan training and the power of a sound Christianity diffused through the community in the midst of which they live and operate. (See Unitarianism).
The Nuttall Encyclopedia 
A designation applicable to all monotheists in religion, including Jews and Mohammedans, but generally and more specially applied to those who deny the Church doctrine of the Trinity, and in particular the divinity of Christ, and who have at different times and in different countries assumed an attitude, both within the pale of the Church and outside of it, of protestation against the opposite orthodox creed in the interests of rationalistic belief; the name is also employed in philosophy to designate those who resolve the manifold of being into the operation of some single principle.