1. provoke or stir up
Example : The use of tear gas set off the protesters.
2. cause to burst with a violent release of energy
Example : The kids set off rockets on the 4th of July.
3. direct attention to, as if by means of contrast 4. put in motion or move to act 5. leave where one is
Example : I set off for London by foot with two quid and a biscuit.
7. set in motion or cause to begin
Example : Mixing Chlorine with Ammonia sets off a dangerous reaction.