Answer:
its a good question.
if you look at wiki definition (well its not necessiailty a source of truth, but it refers largely wat a typical person thinks).....terrorism is defined as use of violence to achieve ones' political goal, which is not really different from activism.
i guess the easy answer is that terrorism is a violent act against the traditional social value; whereas activism is a fight, sometimes violent, against what the society considers as unjustice.
if you want a more thoughtful answer, you can add that sometimes one's definition of activism can be considered as terrorsim by a different person, because different ppl in the society have different social value and priority.
say if some self acclaimed activists run riot on the street, some ppl would consider them as activists; but if you are a store owner who dont agree with their view, their act is an act of terror.
Step-by-step explanation: