Charles Buck Theological Dictionary 
A sect of the Roman Catholics in France who followed the opinions of Jansenius (bishop of Ypres, and doctor of divinity of the universities of Louvain and Douay, ) in relation to grace and predestination. In the year 1640, the two universities just mentioned, and particularly father Molina and father Leonard Celsus, thought fit to condemn the opinions of the Jesuits on grace and free will. This having set the controversy on foot, Jansenius opposed to the doctrine of the Jesuits the sentiments of St. Augustine, and wrote a treatise on grace which he entitled Augustinus. This treatise was attacked by the Jesuits, who accused Jansenius of maintaining dangerous and heretical opinions; and afterwards in 1642, obtained of Pope Urban VIII. a formal condemnation of the treatise wrote by Jansenius; when the partisans of Janenius gave out that this bull was spurious, and composed by a person entirely devoted to the Jesuits. After the death of Urban VIII. the affair of Jansenism began to be more warmly controverted, and gave birth to a great number of polemical writings concerning grace; and what occasioned some mirth, were the titles which each party gave to their writings: one writer published the Torch of St. Augustine; another found Snuffers of St. Augustine's Torch; and father Veron formed A Gag for the Jansenists, &c. In the year 1650, sixty-eight bishops of France subscribed a letter to pope Innocent X. to obtain an inquiry into and condemnation of the five following propositions, extracted from Jansenius's Augustinus:
1. Some of God's commandments are impossible to be observed by the righteous, even though they endeavour with all their power to accomplish them.
2. In the state of corrupted nature, we are incapable of resisting inward grace.
3. Merit and demerit, in a state of corrupted nature, do not depend on a liberty which excludes necessity, but on a liberty which excludes constraint.
4. The Semi-pelagians admitted the necessity of an inward preventing grace for the performance of each particular act, even for the beginning of faith; but they were heretics in maintaining that this grace was of such a nature that the will of man was able either to resist or obey it.
5. It is Semi- pelagianism to say, that Jesus Christ died, or shed his blood, for all mankind in general. In the year 1652, the pope appointed a congregation for examining into the dispute relative to grace. In this congregation Jansenius was condemned; and the bull of condemnation published in May, 1653, pope Alexander VII. issued out another bull, in which he condemned the five propositions of Jansenius. However, the Jansenists affirmed that these propositions were not to be found in this book; but that some of his enemies having caused them to be printed on a sheet, inserted them in the book, and thereby deceived the pope. At last Clement XI. put an end to the dispute by his constitution of July 17, 1705, in which, after having recited the constitutions of the predecessors in relation of this affair, he declared.
"That, in order to pay a proper obedience to the papal constitutions concerning the present question, it is necessary to receive them with a respectful silence." The clergy of Paris, the same year, approved and accepted this bull, and none dared to oppose it. This is the famous bull Unigenitus, so called from its beginning with the words, Unigenitus Dei Filius, &c. which has occasioned so much confusion in France. It was not only on account of their embracing the doctrines of Augustine, that the Jesuits were so imbittered against them; but that which offended the Jesuits, and the other creatures of the Roman pontiff, was, their strict piety, and severe moral discipline. The Jansenists cried out against the corruptions of the church of Rome, and complained that neither its doctrines nor morals retained any traces of their former purity. They reproached the clergy with an universal depravation of sentiments and manners, and an entire forgetfulness of the dignity of their character and the duties of their vocation; they censured the licentiousness of the monastic orders, and insisted upon the necessity of reforming their discipline according to the rules of sanctity, abstinence, and self-denial, that were originally prescribed by their respective founders.
They maintained, also, that the people ought to be carefully instructed in all the doctrines and precepts of Christianity; and that, for this purpose, the Holy Scriptures and public liturgies should be offered to their perusal in their mother tongue; and, finally, they looked upon it as a matter of the highest moment to persuade all Christians that true piety did not consist in the observance of pompous rites, or in the performance of external acts of devotion, but in inward holiness and divine love. Notwithstanding the above-mentioned sentiments, the Jansenists have been accused of superstition and fanaticism; and, on account of their severe discipline and practice, have been denominated Rigourists. It is said, that they made repentance consist chiefly in those voluntary sufferings which the transgressor inflicted upon himself, in proportion to the nature of his crimes and the degree of his guilt. They tortured and macerated their bodies by painful labour, excessive abstinence, continual prayer, and contemplation: nay, they carried these austerities, it is said, to so high a pitch, as to place merit in them, and to consider those as the sacred victims of repentance who had gradually put an end to their days by their excessive abstinence and labour.
Dr. Haweis, however, in his Church History, (vol. 3: p. 46.) seems to form a more favourable opinion of them. "I do not, " says he, "readily receive the accusations that Papists or Protestants have objected to them, as over rigorous and fanatic in their devotion; but I will admit many things might be blameable: a tincture of popery might drive them to push monkish austerities too far, and secretly to place some merit in mortification, which they in general disclaimed; yet, with all that can be said, surely the root of the matter was in them. When I read Jansenius, or his disciples Pascal or Quesnel, I bow before such distinguished excellencies, and confess them my brethren; shall I say my fathers? Their principles are pure and evangelical; their morals formed upon the apostles and prophets; and their zeal to amend and convert, blessed with eminent success."
Watson's Biblical & Theological Dictionary 
a denomination of Roman Catholics in France, which was formed in the year 1640. They follow the opinions of Jansenius, bishop of Ypres, from whose writings the following propositions are said to have been extracted:—
1. That there are divine precepts which good men, notwithstanding their desire to observe them, are, nevertheless, absolutely unable to obey; nor has God given them that measure of grace which is essentially necessary to render them capable of such obedience.
2. That no person, in this corrupt state of nature, can resist the influence of divine grace, when it operates upon the mind.
3. That, in order to render human actions meritorious, it is not requisite that they be exempt from necessity; but that they be free from constraint.
4. That the Semi-Pelagians err greatly, in maintaining that the human will is endowed with the power of either receiving or resisting the aids and influences of preventing grace.
5. That whoever affirms that Jesus Christ made expiation, by his sufferings and death, for the sins of all mankind, is a Semi-Pelagian. Of these propositions, Pope Innocent X, condemned the first four as heretical, and the last as rash and impious. But he did this without asserting that these were the doctrines of Jansenius, or even naming him; which did not satisfy his adversaries, nor silence him. The next pope, however, Alexander VII, was more particular, and determined the said propositions to be the doctrines of Jansenius; which excited no small trouble in the Gallican church.
This denomination was also distinguished from many of the Roman Catholics, by their maintaining that the Holy Scriptures and public liturgies should be given to the people in their mother tongue; and they consider it as a matter of importance to inculcate upon all Christians, that true piety does not consist in the performance of external devotions, but in inward holiness and divine love.
As to Jansenius, it must be confessed that he was more diligent in the search of truth, than courageous in its defence. It is said that he read through the whole of St. Augustine's works ten, and some parts thirty, times. From these he made a number of excerpta, [extracts,] which he collected in his book called "Augustinus." This he had not the courage to publish; but it was printed after his death, and from it his enemies, the Jesuits, extracted the propositions above named; but the correctness and fidelity of their extracts maybe justly questioned. Jansenius himself, undoubtedly, held the opinions of Calvin on unconditional election, though he seems to have been reserved in avowing them.
The Jansenists of Port Royal may be denominated the evangelical party of the Catholic church: among their number were the famous Father Quesnel, Pierre Nicole, Pascal, De Sacy, Duguet, and Arnauld; the last of whom is styled by Boileau, "the most learned mortal that ever lived." They consecrated all their great powers to the service of the cross; and for their attachment to the grand article of the Protestant reformation,—justification by faith, with other capital doctrines, they suffered the loss of all things.
The Jesuits, their implacable enemies, never ceased until they prevailed upon their sovereign, Louis XIV, to destroy the abbey of Port Royal, and banish its inhabitants. It must be confessed, however, that all the Jansenists were not like the eminent men whom we have just mentioned; and even these were tinged with enthusiasm and superstition. Some of them even pretended to work miracles, by which their cause was greatly injured.
The Nuttall Encyclopedia 
A party in the Roman Catholic Church, supporters of Jansen's views, who, in opposition to the Jesuits, maintained the Augustinian principle of the sovereign and irresistible nature of divine grace. The most celebrated members of the party were the Port-Royalists ( q. v .) of France, in particular Arnauld and Pascal, and they were opposed not only by the Jesuits, but by both Louis XIV. and the Pope. Driven from France on the death of Louis, they took refuge in Holland, and thither the Pope Clement XI. followed them, first in 1713, hurling a bull against them, and then in 1719 by ex-communicating them and driving them for good from within the pale of the Catholic Church.