(1) A branch of biology that deals with the functions, mechanisms, and activities of organisms at the cellular or organ system level.
(2) The biological processes and functions of the parts of an organism.
Physiology is a broad sub-field in biology. It may be categorized into animal and plant physiology depending on the organisms described.
Since physiology determines the relative functions of parts, it crosses another important sub-field in biology, which is anatomy.
Word origin: Latin physiologia, from Ancient Greek physis (natural, origin) + -logia (study of)