Watson's Biblical & Theological Dictionary 
The Emperor Charles V called a diet at Spire, in 1529, to request aid from the German princes against the Turks, and to devise the most effectual means for allaying the religious disputes which then raged in consequence of Luther's opposition to the established religion. In this diet it was decreed by Ferdinand, archduke of Austria, and other popish princes that in the countries which had embraced the new religion it should be lawful to continue in it till the meeting of a council; but that no Roman Catholic should be allowed to turn Lutheran, and that the reformers should deliver nothing in their sermons contrary to the received doctrine of the church. Against this decree, six Lutheran princes, namely, John and George, the electors of Saxony and Brandenburg, Ernest and Francis, the two dukes of Lunenburg, the landgrave of Hesse, and the prince of Anhalt, with the deputies of thirteen imperial towns, namely, Strasburg, Ulm, Nuremberg, Constance, Rottingen, Windsheim, Memmingen, Nortlingen, Lindaw, Kempten, Hailbron, Wissemburg, and St. Gall, formally and solemnly protested and declared that they appealed to a general council; and hence the name of Protestants, by which the followers of Luther have ever since been known. Nor was it confined to them; for it soon after included the Calvinists, and has now of a long time been applied generally to the Christian sects, of whatever denomination, and in whatever country they may be found, which have separated from the see of Rome.
Mr. Chillingworth, addressing himself to a writer in favour of the church of Rome, speaks of the religion of the Protestants in the following excellent terms: "Know then, sir, that when I say the religion of Protestants is in prudence to be preferred before yours, on the one side, I do not understand by your religion the doctrine of Bellarmine, or Baronius, or any other private man among you, nor the doctrine of the Sorbonne, of the Jesuits, or of the Dominicans, or of any other particular company among you, but that wherein you all agree, or profess to agree, the doctrine of the council of Trent; so, accordingly, on the other side, by the religion of Protestants, I
do not understand the doctrine of Luther, or Calvin, or Melancthon, nor the confession of Augsburg, or Geneva, nor the catechism of Heidelberg, nor the articles of the church of England; no, nor the harmony of Protestant confessions; but that in which they all agree, and which they all subscribe with a greater harmony, as a perfect rule of faith and action; that is, the Bible. The Bible, I say, the Bible only, is the religion of Protestants. Whatsoever else they believe beside it, and the plain, irrefragable, indubitable consequences of it, well may they hold it as a matter of opinion; but as a matter of faith and religion, neither can they with coherence to their own grounds believe it themselves, nor require belief of it of others, without most high and most schismatical presumption. I, for my part, after a long, and, as I verily believe and hope, impartial, search of the true way to eternal happiness, do profess plainly that I cannot find any rest for the sole of my foot but upon this rock only. I see plainly, and with my own eyes, that there are popes against popes, and councils against councils; some fathers against other fathers, the same fathers against themselves; a consent of fathers of one age against a consent of fathers of another age; traditive interpretations of Scripture are pretended, but there are few or none to be found; no tradition but that of Scripture can derive itself from the fountain, but may be plainly proved either to have been brought in in such an age after Christ, or that in such an age it was not in. In a word, there is no sufficient certainty but of Scripture only for any considering man to build upon. This, therefore, and this only, I have reason to believe. This I will profess; according to this I will live; and for this, if there be occasion, I will not only willingly, but even gladly, lose my life, though I should be sorry that Christians should take it from me. Propose me any thing out of this book, and require whether I believe or no, and, seem it never so incomprehensible to human reason, I will subscribe it with hand and heart, as knowing no demonstration can be stronger than this, God hath said so, therefore it is true. In other things, I will take no man's liberty of judging from him; neither shall any man take mine from me."
Under such views the Bible is held as the only sure foundation upon which all true Protestants build every article of the faith which they profess, and every point of doctrine which they teach; and all other foundations, whether they be the decisions of councils, the confessions of churches, the prescripts of popes, or the expositions of private men, are considered by them as sandy and unsafe, or as in nowise to be ultimately relied on. Yet, on the other hand, they by no means fastidiously reject them as of no use; for while they admit the Bible, or the Scriptures of the Old and New Testament, to be the only infallible rule by which we must measure the truth or falsehood of every religious opinion, they are sensible that all men are not equally fitted to understand or to apply this rule; and that the wisest men want, on many occasions, all the helps afforded by the learning and research of others to enable them to understand its precise nature, and to define its certain extent. These helps are great and numerous, having been supplied, in every age of the church, by the united labours of learned men in every country, and by none in greater abundance than by those in Protestant communions.
Webster's Dictionary 
(1): ( a.) Making a protest; protesting.
(2): ( a.) Of or pertaining to the faith and practice of those Christians who reject the authority of the Roman Catholic Church; as, Protestant writers.
(3): ( v.) One who protests; - originally applied to those who adhered to Luther, and protested against, or made a solemn declaration of dissent from, a decree of the Emperor Charles V. and the Diet of Spires, in 1529, against the Reformers, and appealed to a general council; - now used in a popular sense to designate any Christian who does not belong to the Roman Catholic or the Greek Church.
Charles Buck Theological Dictionary 
A name first given in Germany to those who adhered to the doctrine of Luther, because in 1529, they protested against a decree of the emperor Charles V. and the diet of Spires; declaring that they appealed to a general council. The same has also been given to those of the sentiments of Calvin; and is now become a common denomination for all those of the reformed churches.