also half life, 1864, "unsatisfactory way of living," from half + life; the sense in physics, "amount of time it takes half a given amount of radioactivity to decay" is first attested 1907.

1832 in mathematics and physics, "a quantity which is assumed to be invariable throughout," from constant (adj.), which is attested from 1753 in mathematics. The general sense "that which is not subject to change" (1856) is a figurative extension from this.

in physics, in reference to the work of German physicist Max Planck (1858-1947); such as Planck's constant, attested in English from 1901.

also destabilise, "deprive of stability," 1928 in a chemistry and physics sense; earlier (1919) with reference to political systems, commercial markets, etc.; see de- + stabilize. Related: Destabilized; destabilizing.

1610s, "removal from office;" see displace + -ment. As "quantity of a liquid displaced by a solid body put into it," 1809. Physics sense "amount by which anything is displaced" is from 1837.

c. 1600, "quality of being very close or compact," from French densité (16c.), from Old French dempsité (13c.), from Latin densitas "thickness," from densus "thick, dense" (see dense). In physics, "the mass of matter per unit of bulk," 1660s.

1520s, "having the power or property of leading" (a sense now obsolete), from conduct (v.) + -ive. Physics sense, "resulting from or pertaining to conduction," is from 1840. Related: Conductivity (1837).

"physical structure or organization," especially of a human body, 1826, from French physique, noun use of physique (adj.) "physical," from Latin physicus "natural, physics," from Greek physikos, from physis "nature" (from PIE root *bheue- "to be, exist, grow").

as a branch of physics that calculates motions in accordance with the laws of force, by 1752, from dynamic (adj.); also see -ics. As "the moving physical or moral force in anything," by 1833.