Definition: Haganah is the Hebrew word for "The Defense." Haganah was a Jewish paramilitary force established in 1920 in Palestine during the British Mandate, when Jews concluded that the British would not seriously confront Arabs who attacked Jewish settlements. The Haganah was established to protect Jewish farms and Kibbutzim.
The Haganah was itself the successor of looser, more informal guard corps or militias: The Bar-Giora, created by immigrants in 1907 as a enterprise-gang that guarded settlements for cash, followed by the Hashomer, or the "Guild of Watchmen," created two years later. The Haganah, in turn, gave rise to the institutionalized Israeli military in 1948, known as the Israeli Defense Forces, or IDF.