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.
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.
"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").