From the Ency. Judica 16:947

The name, recorded by the Jewish historian Joephus, by which some Jewish resistance fighters were known in the Jewish War, 66-73 C.E. This name is important in view of it's portable origin.
Meaning of the Name. Josephus provides a clue to it's real meaning both by his apparent reluctance to use it, and by his explanation of it when he does. This, when his narrative obliged him to record that the Jewish insurgents who seized the Temple in 66 C.E. were known as Zealots, he adds: "for so these miscreants called themselves, as though they were zealots in the cause of virtue and not for vice in it's based and most extravagant form" (Wars, 4:161; cf. 7:268-70).
By this denigrating comment Josephus reveals that the name Zealot was not merely descriptive of the character and action of the Jewish resistance party, but that it had been deliberately adopted by the party itself.
A passage from the Sanhedrin (9:6 ; TB Sanh. 82a) provides a further clue to the orginal meaning by associating the Zealots (Kanna'im) with action that recalls the deed of Phinehas in
Numbers 25:7-13 . Phinehas is praised for hie zeal (be-Kano) in defending God's cause, thus becoming the divinely commanded prootype of the Zealot, who was ready to resort to violent action to defend the honor of God and protect the Torah.
Josephus, who defected to the Romans and blamed the Zealots for the catastrophe of 70 C.E., was obviously reluctant to call them by the name which proclaimed their religious inspiration. He preferred to describe them as (lestai , "brigands") or as ( sicarii, "dagger-men").
His indiscriminate use of the latter term for the Zealots is particularly confusing, for it would seem that he purposely called the main resistance party by an obbrobrious term more appropriate to the terrorist organization.

A careful analysis of Josephus' writings reveals that the Zealot movement stemmed from the action of Judah the Galilean in 6 C.E. and embodied the fundamental principles of his teaching. In that year Judea was for the first time, incorporated into the Roman Empire, and P. Sulpicus Quirinius, the legate of Syria, was ordered of taxing the new possession. The high priest Joazar counseled the people to submit to the Roman requirement; but Judah the Galilean, who was probably a rabbi, assisted by Zadok, a Pharisee, urged resistance.
In Josephus' writing from the "Jewish Wars" about 75-79 C.E. clearly staes the two principles of Judah's teaching: that the Jews should not pay tribute to Rome, or acknowledge the emperor, a mortal man, as their master. These two principles were interrelated, and indicate that Judah conceived of Israel as a theocracy. The objection to the Roman tribute evidently derived from this conception. Judea was the Holy Land, and to give of it's resoures in tribute to the Romans was an act of apostasly.

Joesphus ends "Antiquites" (completed 93-94 C.E.) by blaming the followers of Judah for the destruction of the Temple. In a further descriptive note (Ant. 18:23-25), in which he still studiously refrains from using the name Zealot, he says that the followers of Judah associated themselves generally with the Pharisees, and that "they have a passion for liberty that is almost unconquerable, since they are convinced that God alone is their leader and master," and he comments on their amazing fortitude toward suffering.

Zealots Part 2