1841, "of or like the nucleus of a cell," from nucleus + -ar, probably by influence of French nucléaire. General sense of "central" is from 1912. In atomic physics, "of or belonging to the nucleus of an atom," from 1914; of weapons deriving their destructive power from nuclear reactions, by 1945.
Hence nuclear energy (1930), nuclear physics (1933), nuclear war (1954). Nuclear winter was coined by U.S. atmospheric scientist Richard Turco but is first attested in article by Carl Sagan in "Parade" magazine, Oct. 30, 1983. Nuclear family, originally a sociologists' term, is first attested 1949 in "Social Structure," by American anthropologist G.P. Murdock (1897-1985). Alternative adjective nucleal is recorded from 1840, probably from French.
"act of refracting; state of being refracted," 1570s, from Late Latin refractionem (nominative refractio) "a breaking up," noun of action from past-participle stem of Latin refringere "to break up," from re- "back" (see re-) + combining form of frangere "to break" (from PIE root *bhreg- "to break"). According to Century Dictionary, "Almost exclusively restricted to physics" .
1540s, "a leading, guidance" (a sense now obsolete), from French conduction "hire, renting," and directly from Latin conductionem (nominative conductio), noun of action from past-participle stem of conducere "to lead or bring together," from assimilated form of com "with, together" (see con-) + ducere "to lead" (from PIE root *deuk- "to lead").
Sense of "a conducting through a channel" is from 1610s in reference to liquids; in physics, "transmission, conveyance" of heat, etc., from 1814.
"to synchronize, adjust the phase of so as to synchronize," 1895, from phase (n.) in the physics sense of "particular stage or point in a recurring sequence of movement or changes" (1861). Earlier as a bad spelling of faze. Meaning "to carry out gradually" is from 1949, hence phase in "introduce gradually" (1954), phase out "take out gradually in planned stages" (1954). Related: Phased; phasing.