This word refers to physiological changes that occur within the body as a response to environmental stresses.  In other words, the body adapts to being subjected to environmental extremes like hot, cold, etc.  An example of this would be the improvement that occurs in the sweating mechanism as a result of training in hotter climates.  When training for an event, keep this physiological concept in mind.  If you know you are training for the Ironman in Kona, Hawaii, then you may want to try to train in some extremely hot conditions so your body will be ready.  

This is an easy concept to grasp, but one that is often overlooked during training, and one that can make a difference!