Allostasis is the process of achieving stability, or homeostasis, through physiological or behavioral change. This can be carried out by means of alteration in HPA axis hormones, the autonomic nervous system, cytokines, or a number of other systems, and is generally adaptive in the short term (McEwen & Wingfield, 2003). The key difference between allostatis and homeostatis is described in Robert Sapolsky's book Why Zebras don't get ulcers:

Homeostatis is the regulation of the body to a balance, by single point tuning. Too much water? Adjust kidney processing.

Allostatis is adaptation to a more dynamic balance. Sweat less, change water use, change water retention, change chemical processing, and so on, to meet this water-need at this time, which may be a different balance than the best water balance at another time.