What is the difference between allostasis and homeostasis?

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To me, the two terms mean the same thing- internal biochemical and physiological balance. However, many scientist claim that the two are distinctly different. In the process of trying to get to the root difference, I came across a new word (heterostasis) which is also claimed to be different from the above two terms (allostasis and homeostasis) yet to me, it also mean internal regulation.

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