Acclimatization is the physical mechanisms by which a body adapts to new condition, such as light, temperature, and altitude. If you have ever moved to someplace that was extremely different from the place you had moved from you have doubtless experienced a period of discomfort before you finally "got used to," or acclimatized to your new environment.
For example, when adjusting to a different altitude, acclimatization occurs through subtle changes in the body such as lung capacity variations or a thickening or thinning of the blood to accommodate the oxygen content of the air. These adaptations are necessary to accommodate the new environment. Acclimatization is not a permanent state in that it is adaptive - if you return to your 'normal' altitude your body would adapt again in order to deal with the environmental changes.