The Zealots were fanatical, militant Jews who opposed Roman rule in the first century A.D. Zealots used military force to fight against the Romans and the Jews who complied with them. According to Luke 6:15, the Disciple Simon was a member of the Zealots.
The group's activities mostly ceased after the Romans captured the Jewish hill-top fortress Masada in A.D. 73, after a two-month siege. According to Jewish historian Josephus, all but two of the over one thousand zealots killed one another rather than submit to the Romans.
Nowadays, the word zealot is applied to anyone with fanatical religious convictions.