One man's terrorist is another man's freedom fighter.
From a personal point of view i would say a group that targets military or infrastructure avoiding innocent civilian casualties are freedom fighters(Hezbollah since 1999,the original IRA in the early part of the 20th century,and the ANC in south Africa are all examples).
Groups that target civilians are terrorists,period.
As for Irgun and the Stern gang,they were both Zionist groups looking to remove an imperial power(to the point of prefering to fight the British rather than Fascism during WW2),unfortunately they also removed by force most of the people living in that land,that makes them terrorists IMO.