The term Zealot, in Hebrew kanai means one who is jealous on behalf of God, meaning anyone who is overly zealous.
Jewish history
War with Rome
Although today it means anyone who is overly zealous, it originally referred to a Jewish political movement in the first century CE that sought to incite the people of Judea to rebel against the Roman Empire and expel it from the country by force of arms during the Great Jewish Revolt.
Other important Jewish factions during the wars against Rome, were the Pharisees, the Sadducees, and the Essenes.
The Zealots had the leading role in the Jewish Revolt of CE 66. They succeeded in taking Jerusalem, and held it until 70, when the son of Roman emperor Vespasian, Titus Flavius, recaptured the city and destroyed the Second Temple during the destruction of Jerusalem.
The Zealots were opposed to Roman rule and sought to eliminate it by violent means. Their activities were limited to raids on Jewish settlements and eliminating Jewish collaborators.
Josephus
The ancient Jewish historian and leader, Josephus, has written of his experiences during the struggle with Rome and about his attempts to save Jerusalem from the Zealots.
The Talmud
In the Talmud, the Zealots are also called the baryonim meaning "boorish" or "wild", and are condemned for their aggression, unwillingness to compromise to save the living survivors of Jerusalem besieged by the Romans, and blind-militarism and are blamed for having contributed to the demise of Jerusalem, the second Jewish Temple and of ensuring Rome's retributions and stranglehold on Judea.
Masada
One of their leaders, Elazar ben Yair managed to escape to the desert fortress of Masada and fought alongside the Sicarii Zealots until Masada was captured in 73.
The Jewish Revolt was quickly suppressed and the Zealots lost all their influence and finally vanished.
Sicarii
One particularly extreme group of Zealots was also known in Latin as sicarii, "daggermen" (sing. sicarius), because of their policy of assassinating Jews opposed to their call for war against Rome.
Early Christians
Early Christianity
The Christians movement began during this latter period, at around the Second Jewish Temple's demise.
New Testament narrative
The New Testament was written much later but its narrative is set during those times. Some have speculated that the name of Jesus' disciple Judas Iscariot means that he was a sicarii, "daggermen" (sing. sicarius is a corruption of this term) - "Judas the Zealot". However, the Latin and Hebrew words for "zealot" sound very different, so it is hard to conclude definitively.
Tax collectors, like Matthew, were often collaborating with the Romans.
Among the Apostles of Jesus, there were two possible Zealots, Judas Iscariot and Simon the Canaanite, also known by Luke as "Simon the Zealot".