Charles Buck Theological Dictionary 
A term in the Jewish theology, signifying a work on the Bible, performed by several learned rabbins to secure it from any alterations which might otherwise happen. Their work regards merely the letter of the Hebrew text, in which they have first fixed the true reading by vowels and accents; they have, secondly, numbered not only the chapters and sections, but the verses, words, and letters of the text: and they find in the Pentateuch 5245 verses, and in the whole Bible 23, 206. The masora is called by the Jews, the hedge or fence of the law, because this enumeration of the verses, &c. is a means of preserving it from being corrupted and altered. They have, thirdly, marked whatever irregularities occur in any of the letters of the Hebrew text; such as the different size of the letters, their various positions and inversions, &c. and they have been fruitful in finding out reasons for these mysteries and irregularities in them. They are, fourthly, supposed to be the authors of the Keri and Chetibh, or the marginal corrections of the text in our Hebrew Bibles.
The text of the sacred books, it is to be observed, was originally written without any breaks or divisions into chapters or verses, or even into words: so that a whole book, in the ancient manner, was but one continued word: of this kind we have still several ancient manuscripts, both Greek and Latin. In regard, therefore, the sacred writings had undergone an infinite number of alterations; whence various readings had arisen, and the original was become much mangled and disguised, the Jews had recourse to a canon, which they judged infallible, to fix and ascertain the reading of the Hebrew text; and this rule they call masora; "tradition, " from tradit, as if this critique were nothing but a tradition which they had received from their forefathers. Accordingly they say, that, when God gave the law to Moses at Mount Sinai, he taught him first the true reading of it; and, secondly, its true interpretation; and that both these were handed down by oral tradition from generation to generation, till at length they were committed to writing. The former of these, viz. the true reading, is the subject of the masora; the latter, or true interpretation, that of the mishna and gemera. According to Elias Levita, they were the Jews of a famous school at Tiberias, about five hundred years after Christ, who composed, or at least began, the masora; whence they are called masorites and masoretic doctors.
Aben Ezra makes them the authors of the points and accents in the Hebrew text, as we now find it, and which serve for vowels. The age of the masorites has been much disputed. Archbishop Usher places them before Jerome; Capel, at the end of the fifth century; father Morin, in the tenth century. Basuage says, that they were not a society, but a succession of men; and that the masora was the work of many grammarians, who, without associating and communicating their notions, composed this collection of criticisms on the Hebrew text. It is urged, that there were masorites from the time of Ezra and the men of the great synagogue, to about the year of Christ 1030: and that Ben Asher and Ben Naphtali, who were the best of the profession, and who, according to Basnage, were the inventors of the masora, flourished at this time. Each of these published a copy of the whole Hebrew text, as correct, says Dr. Prideaux, as they could make it. The eastern Jews have followed that of Ben Naphtali, and the western that of Ben Asher: and all that has been done since is to copy after them, without making any more corrections, or masoretical criticisms. The Arabs have done the same thing by their Koran that the Masorites have done by the Bible; nor do the Jews deny their having borrowed this expedient from the Arabs, who first put it in practice in the seventh century. There is a great and little masora printed at Venice and at Basil, with the Hebrew text in a different character. Buxtorf has written a masoretic commentary which he calls Tiberias.
Webster's Dictionary 
(n.) A Jewish critical work on the text of the Hebrew Scriptures, composed by several learned rabbis of the school of Tiberias, in the eighth and ninth centuries.
Holman Bible Dictionary 
Text And Versions Bible