Effectively means dispersal - the Twelve Tribes (some accounts say Ten) who went abroad fol owing the period when the Jews were taken into captivity by the Babylonians through to the period of the Roman occupation.
Diaspora is where you have a mass-migration of a people out of their homeland. The term, as far as it relates to the Jewish people, originated with the Babylonian Exile when many Jews were forced to leave Israel to go to Babylon.