From 1832 in reference to "intelligent study and appreciation of the classics," especially in reference to the Renaissance. By 1847 in reference to "system or mode of thought in which human interests predominate" (originally often in the writings of its enemies). As a pragmatic system of thought, defined 1907 by co-founder F.C.S. Schiller as "The perception that the philosophical problem concerns human beings striving to comprehend a world of human experience by the resources of human minds."
c. 1300, seculer, in reference to clergy, "living in the world, not belonging to a religious order," also generally, "belonging to the state" (as opposed to the Church), from Old French seculer, seculare (Modern French séculier) and directly from Late Latin saecularis "worldly, secular, pertaining to a generation or age," in classical Latin "of or belonging to an age, occurring once in an age," from saeculum "age, span of time, lifetime, generation, breed."
This is from Proto-Italic *sai-tlo-, which, according to Watkins, is PIE instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. De Vaan also connects it with "bind" words and lists as a cognate Welsh hoedl "lifespan, age." An older theory connected it to words for "seed," from PIE root *se- "to sow" (see sow (v.), and compare Gothic mana-seþs "mankind, world," literally "seed of men").
The ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years). Ecclesiastical writers in Latin used it as those in Greek did aiōn "of this world" (see cosmos). It is the source of French siècle "century." The meaning "of or belonging to an age or a long period," especially occurring once in a century, was in English from 1590s.
From mid-14c. in the general sense of "of or belonging to the world, concerned in earthly more than in spiritual, life;" also of literature, music, etc., "not overtly religious." In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s. Related: Secularly.
"exclusive attention to the present life and its duties; doctrine that morality should be based on the well-being of man in the present life, without regard to religious belief or a hereafter," 1843, from secular + -ism.
Secularity (late 14c., seculerte ) was used for "civil or temporal power, the business of layman life; quality or condition of being secular" (from Old French secularite and Medieval Latin sæcularitas).
c. 1300, "secular tribunal," from Old North French consistorie (Old French consistoire, 12c.) and directly from Late Latin consistorium "waiting room, meeting place of the imperial council," from Latin consistere (see consist). Meaning "Church council, an ecclesiastical court" is from early 14c. Related: Consistorial.