What is the X in X-mas?
1560s, "study and description of natural objects," from Middle French physiologie or directly from Latin physiologia "natural science, study of nature," from Greek physiologia "natural science, inquiry into nature," from physio- "nature" (see physio-) + logia "study" (see -logy). Meaning "science of the normal function of living things" is attested from 1610s. Related: Physiologic; physiologist.
physiology phys·i·ol·o·gy (fĭz'ē-ŏl'ə-jē)
The biological study of the functions of living organisms and their parts.
All the functions of a living organism or any of its parts.
The scientific study of an organism's vital functions, including growth and development, the absorption and processing of nutrients, the synthesis and distribution of proteins and other organic molecules, and the functioning of different tissues, organs, and other anatomic structures. Physiology studies the normal mechanical, physical, and biochemical processes of animals and plants.