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.
"apparent displacement of an object observed, due to an actual displacement of the observer," 1570s, from French parallaxe (mid-16c.), from Greek parallaxis "change, alteration, inclination of two lines meeting at an angle," from parallassein "to alter, make things alternate," from para- (see para- (1)) + allassein "to change," from allos "other" (from PIE root *al- "beyond"). Related: Parallactic.
"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."
"mental view or survey," 1742, from out- + look (v.). The meaning "prospect for the future" is attested from 1851. Earliest sense was "a place from which an observer looks out or watches anything" (1660s). The literal sense of "vigilant watch, act or practice of looking out" (1815) is rare; look-out being used instead for this.
1640s, "right as opposed to left," from Medieval Latin dexteralis "on the right," from Latin dexter "right, opposite of left," from PIE root *deks-. From 1871 as "right-handed." By 1818 in reference to univalve shells, "having the aperture on the right side when held upright in front of the observer with the apex upward." Related: Dextrally; dextrality.
"one who observes flights of birds for the purpose of taking omens," 1590s, from Latin auspex "interpreter of omens given by birds," from PIE *awi-spek- "observer of birds," from root *awi- "bird" + root *spek- "to observe." Compare Greek oionos "bird of prey," also "bird of omen, omen," and ornis "bird," which also could mean "omen."