Acclimation is the act of getting used to one's environment. Questions about acclimation are typically along the lines of getting a pet used to it's new home.
Acclimation is a term most commonly used in fish-keeping, although it could be used in other areas. It's the act of getting used to one's environment.
When bringing a fish home, it's important to acclimate it to the tank, to get the fish used to the new water parameters. Likewise, when bringing other animals home, it is important to acclimate them to the new sights, sounds, and smells.