A strategy that utterly dominates another strategy, leaving no question of the outcome.
Compare to soft counter. In a soft counter, the strategy being countered can still be victorious through skill or luck. There is no chance of this when a hard counter is used.
1) Rock-paper-scissors: Rock hard counters scissors, scissors hard counters paper, etc. There is no situation in which scissors will beat rock.
2) In most strategy games, tanks will hard-counter infantry without anti-tank weapons. A rifleman with an M1 stands no chance whatsoever against a Kraut riding the MG42 on top of a Tiger.