Adaptation are changes in an organism's physiological structure or function or habits that allow it to survive in new surroundings.

Other definition:
An Adaptation is an adjustment made by animals in respect of their environments. The adjustments may occur by natural selection, as individuals with favorable genetically acquired traits breed more prolifically than those lacking these traits (genotypic Adaptation), or they may involve non-genetic changes in individuals, such as physiological modification (e.g. acclimatization) or behavioral changes,

Other definition:
Adaptation is any genetically controlled structural, physiological, or behavioral Characteristic that helps an Organism survive and reproduce under a given set of environmental conditions. It usually results from a beneficial mutation. See biological evolution, differential reproduction, mutation, natural selection.