Definition
 noun
 (1) The outside of the cells inside the body of an organism.
 (2) The extracellular fluid environment.
 Supplement
 The internal environment refers to the internal milieu of a multicellular organism. Every cell of a multicellular organism must regulate its surroundings to help keep a relatively stable internal environment.
 Synonym:
Compare: external environment
 
See also: homeostasis.