mid-15c., "deterioration, decline in value, gradual loss of soundness or perfection," from decay (v.). Obsolete or archaic in reference to fortune or property; meaning "decomposition of organic tissue" is from 1590s. In physics, the meaning "gradual decrease in radioactivity" is by 1897.
in physics, the metric unit of force, 1873, from a specialized scientific use of of Greek dynamis "power" (see dynamic (adj.)); perhaps also influenced by French dyne, which had been proposed c. 1842 as a unit of force in a different sense.
in physics, 1896, in Roentgen rays "X-rays," in recognition of German physicist Wilhem Conrad Röntgen (1845-1923), who discovered X-rays in 1895. As a unit of exposure to radiation, it is attested from 1922, proposed in French in 1921.
1560s, "study and description of natural objects, natural philosophy" (a sense now obsolete), from French physiologie (16c.) or directly from Latin physiologia "natural science, study of nature," from Greek physiologia "natural science, inquiry into nature," from physios "nature" (see physio-) + logia "study" (see -logy). Meaning "science of the normal function of living things" is attested from 1610s. Related: Physiologic; physiologist.
The two words [physics/physiology] had once the same wide meaning of natural science or natural philosophy. They have now been narrowed & differentiated, physics retaining only the properties of matter & energy in inorganic nature, & physiology only the normal functions & phenomena of living beings. [Fowler, 1926]
early 15c., "incidental matter," from Old French incidence (15c.), from Late Latin incidentia, from incidere "to happen, befall" (see incident (n.)). Meaning "act of coming into contact with or affecting" is from 1650s. In physics, of rays of light, etc., considered with reference to direction, from 1620s.
by 1858 as "one who maintains that human intellectual and moral nature depend on and results from one's physical constitution or organization," from physical (adj.) + -ist. By 1934 as "one who holds the theory that all science must be capable of being expressed in the language of physics." Related: Physicalism.