c. 1600, "that must be attended, followed, or kept," also "worthy of notice or mention," from Latin observabilis "remarkable, observable," from 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." From 1640s as "perceptible, capable of being observed." Related: Observably; observability.

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.

late 14c., observacioun, "the performance of a religious rite," from Old French observation (c. 1200) and directly from Latin observationem (nominative observatio) "a watching over, observance, investigation," noun of action from past-participle stem 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." Sense of "act or fact of paying attention" is from 1550s. Meaning "a remark in reference to something observed" is recorded from 1590s.

1834, "of or pertaining to (scientific) observation," from observation + -al (1).

"of or pertaining to observation," 1610s, from Latin observat-, past-participle stem of observare "watch over, note, heed, look to, attend to, guard, regard, comply with" (see observe) + -ive.

"building for observing astronomical phenomena," 1670s (in reference to Greenwich), from French observatoire, from observer (v.) "to observe, watch over, follow," from Latin observare "watch over," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect."

late 14c., observen, "to hold to (a manner of life or course of conduct), carry out the dictates of, attend to in practice, to keep, follow," from Old French observer, osserver "to observe, watch over, follow" (10c.), from Latin 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." Sense of "watch, perceive, notice" is from 1560s, via the notion of "see and note omens." Meaning "to say by way of remark" is from c. 1600. Related: Observed; observing.

1550s, "one who keeps a rule, custom, etc.," agent noun from observe. Meaning "one who watches and takes notice" is from 1580s; this is the sense of the word in many newspaper names. Meaning "one who observes without participating" (at a meeting, conference, etc.) is by 1925.

c. 1500, "to besiege" (a sense now obsolete), from Latin obsessus, past participle of obsidere "watch closely; besiege, occupy; stay, remain, abide" literally "sit opposite to," from ob "against" (see ob-) + sedere "to sit," from PIE root *sed- (1) "to sit." Of evil spirits, "to haunt," from 1530s. The psychological senseof "to haunt as a fixed idea" developed gradually from 1880s and emerged 20c. The 1895 Century Dictionary has only the two senses "to besiege" (marked obsolete) and "to attack, vex, or plague from without." Related: Obsessed; obsessing.