mid-13c., observaunce, "act performed in accordance with prescribed usage," especially a religious or ceremonial one; late 14c., "care, concern, act of paying attention (to something)," from Old French observance, osservance "observance, discipline," and directly from Latin observantia "act of keeping customs, attention, respect, regard, reverence," from observantem (nominative observans), present participle of observare "watch over, note, heed, look to, attend to, guard, regard, comply with," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect." Observance is the attending to and carrying out of a duty or rule. Observation is watching, noticing.
c. 1600, "attentive in perceiving or taking notice, characterized by good powers of observation," also "attentive in observing what is prescribed or required" (a law, custom, etc.), from observe + -ant, or else from French observant, past participle of observer (see observance). In reference to Judaism, "strict in acting in accordance with precepts," from 1902. As a noun from late 15c. Related: Observantly; observantness.
It is the hypothetical source of/evidence for its existence is provided by: Avestan haurvaiti "to guard;" Latin servare "to guard, keep, watch;" Old Church Slavonic xraniti "to guard, protect;" Old High German gi-sarwi "armor, equipment," Old English searu "art, skill; wile, deceit."
"pertaining to the Sabbath or its observance," 1630s, from Latin sabbatarius (adj.), from sabbatum (see Sabbath).
"promoting orderly observance of rules," 1590s, from Medieval Latin disciplinarius, from Latin disciplina "instruction given, teaching," also "military discipline" (see discipline (n.)).
early 14c., "formal act or procedure of religious observance performed according to an established manner," from Latin ritus "custom, usage," especially "a religious observance or ceremony" (source also of Spanish, Italian rito), which perhaps is from PIE root *re- "to reason, count," on the notion of "to count; to observe carefully." Rite of passage (1909), marking the end of one phase and the start of another in an individual life, is translated from French rite de passage, coined by French anthropologist Arnold van Gennep (1873-1957).
"of or pertaining to the Pharisees," hence "of or pertaining to observance of the external forms and ceremonies of religion without regard to its spirit or essence," 1610s, from Church Latin pharisaicus, from Greek pharisaikos, from pharisaios (see Pharisee). Related: Pharisaical (1530s).
"very nice or precise in behavior, exact in the observance of rules or forms prescribed by law or custom," 1630s, probably from Italian puntiglioso, from puntiglio "fine point," from Latin punctum "prick" (from nasalized form of PIE root *peuk- "to prick"). Related: Punctiliously; punctiliousness.