Acclimation refers to:
Acclimation is commonly defined as a reversible physiological adjustment by an individual to one controlled environmental variable in the laboratory. For example, animals maintained at a new experimental temperature may alter enzyme expression, membrane lipid composition, metabolic rate, or thermal tolerance over days or weeks. Acclimatization usually refers to a comparable adjustment under natural conditions, where temperature, humidity, oxygen, and other variables may change together. Both processes occur within a lifetime and depend on phenotypic plasticity; neither requires a new mutation
Ref: Evolutionary Analysis, Herron & Freeman, 5th Ed., Ch. 10