Which word is opposite to "warm-up" in sports? I mean slow running, gentle exercise after hard work to accelerate recovery processes.
Cooling down. A cool down is an easy exercise that will allow the body to gradually transition to a resting or near-resting state.
When you workout you should always incorporate a warm up before and a cool down afterwards.
Warm-down is broadly used especially in running or racing as indicated in the below definition:
a tapering off or recovery from strenuous physical exercise, especially running or racing, by slowing down or doing light stretches.