Adaptation is a term in fitness and sports that refers to the way the body programs the muscles to remember particular activities, movements or skills.
By repeating that skill or activity, the body adapts to the stress and the skill becomes easier to perform. Adaptation explains why beginning exercisers are often sore after starting a new routine, but after doing the same exercise for weeks and months the athlete has little, if any, muscle soreness. This also explains the need to vary the routine and continue to apply the Overload Principle if continued improvement is desired. The levels of adaptation are quite complex.
Other definition:
Adaptation refers to the ability of the body to adjust; for example, the capacity to adjust to cold or hot climates.