mid-14c., "specially prepared or arranged display," from Old French spectacle "sight, spectacle, Roman games" (13c.), from Latin spectaculum "a public show, spectacle, place from which shows are seen," from spectare "to view, watch, behold," frequentative form of specere "to look at" (from PIE root *spek- "to observe").
It forms all or part of: aspect; auspex; auspices; auspicious; bishop; circumspect; conspicuous; despicable; despise; episcopal; especial; espionage; espy; expect; frontispiece; gyroscope; haruspex; horoscope; inspect; inspection; inspector; introspect; introspection; perspective; perspicacious; perspicacity; prospect; prospective; respect; respite; retrospect; scope; -scope; scopophilia; -scopy; skeptic; species; specimen; specious; spectacle; spectacular; spectrum; speculate; speculation; speculum; spice; spy; suspect; suspicion; suspicious; telescope.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit spasati "sees;" Avestan spasyeiti "spies;" Greek skopein "behold, look, consider," skeptesthai "to look at," skopos "watcher, one who watches;" Latin specere "to look at;" Old High German spehhon "to spy," German spähen "to spy."
"demonstrable proposition in science or mathematics," 1550s, from French théorème (16c.) and directly from Late Latin theorema, from Greek theorema "spectacle, sight," in Euclid "proposition to be proved," literally "that which is looked at," from theorein "to look at, behold" (see theory).
1796, "a painting on a revolving cylindrical surface," representing scenes too extended to be beheld all at once, coined c. 1789 by inventor, Irish artist Robert Barker, literally "a complete view," from pan- "all" + Greek horama "sight, spectacle, that which is seen," from horan "to look, see," which is possibly from PIE root *wer- (3) "to perceive, observe." Meaning "comprehensive survey, complete or entire view" is by 1801.