taken as a measure against something possible, anticipated, or feared; preventive; deterrent: a preemptive tactic against a ruthless business rival.
3.
preempting or possessing the power to preempt; appropriative; privileged: a commander's preemptive authority.
4.
Bridge. pertaining to, involving, or noting an opening bid or an overcall in a suit that is at an unnecessarily high level and that is essentially a defensive maneuver designed to make communication between one's opponents more difficult: a preemptive bid; to give a preemptive response.
1855, also preemptive, "pertaining to preemption;" from preempt + -ive. Specifically of an attack on an enemy who is plotting his own attack, from 1959, a term from the Cold War.