early 14c., "deterioration, degeneration, a sinking into an impaired or inferior condition," from Old French declin, from decliner "to sink, decline, degenerate" (see decline (v.)). Meaning "the time of life when physical and mental powers are failing" is short for decline of life (by 1711).

late 14c., "to turn aside, deviate" (a sense now archaic), also "sink to a lower level," and, figuratively, "fall to an inferior or impaired condition," from Old French decliner "to sink, decline, degenerate, turn aside," from Latin declinare "to lower; avoid, deviate; bend from, inflect," from de "from" (see de-) + clinare "to bend" (from PIE *klein-, suffixed form of root *klei- "to lean").

In grammar, "to inflect as a noun or adjective," from late 14c. The sense has been altered by interpretation of de- as "downward;" intransitive meaning "to bend or slant down" is from c. 1400. Sense of "not to consent, politely refuse or withhold consent to do" is from 1630s. Related: Declined; declining.

late 14c., declinacioun, in astronomy, "distance of a heavenly body from the celestial equator, measured on a great circle passing through the body and the celestial pole," from Old French declinacion (Modern French déclinaison) and directly from Latin declinationem (nominative declinatio) "a bending from (something), a bending aside; the supposed slope of the earth toward the poles; a turning away from (something), an avoiding," noun of action from past-participle stem of declinare (see decline (v.)). From c. 1400 as "a bending or sloping downward." Related: Declinational.

Proto-Indo-European root meaning "to lean."

It forms all or part of: acclivity; anticline; clemency; client; climate; climax; cline; clinic; clinical; clino-; clitellum; clitoris; decline; declivity; enclitic; heteroclite; incline; ladder; lean (v.); lid; low (n.2) "small hill, eminence;" matroclinous; patroclinous; polyclinic; proclitic; proclivity; recline; synclinal; thermocline.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian šlyti "to slope," šlieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting").

"a decline," 1926 in an economic sense, from the prepositional phrase; see down (adv.) + turn (n.).

by 1807 as "after the lifetime of Christ," from post- + Christ + -ian; by 1929 as "after the decline or rejection of Christianity," from Christian.

late 14c., recusen, "to decline, refuse," especially "reject another's authority or jurisdiction over oneself as prejudiced," from Old French recuser (13c.) and directly from Latin recusare "make an objection against; decline, refuse, reject; be reluctant to," from re- (see re-) + causa (see cause (n.)). Specifically, in law, "reject or challenge (a judge or juror) as disqualified to act." The word now is used mostly reflectively. Related: Recused; recusing; recisative.