Physiology. Definition. noun. (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. Supplement. Physiology is a broad sub-field in biology.hysiology. Definition. noun. (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. Supplement. Physiology is a broad sub-field in biology.