Etymology
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oculus (n.)
"an eye," plural oculi, 1857, from Latin oculus "an eye" (from PIE root *okw- "to see").
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*okw- 

Proto-Indo-European root meaning "to see."

It forms all or part of: amblyopia; antique; antler; atrocity; autopsy; binocle; binocular; biopsy; catoptric; Cyclops; daisy; enoptomancy; eye; eyelet; ferocity; hyperopia; inoculate; inveigle; monocle; monocular; myopia; necropsy; ocular; oculist; oculus; oeillade; ogle; ophthalmo-; optic; optician; optics; optometry; panoptic; panopticon; Peloponnesus; pinochle; presbyopia; prosopopeia; stereoptican; synopsis; triceratops; ullage; wall-eyed; window.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit akshi "the eye; the number two," Greek osse "(two) eyes," opsis "a sight;" Old Church Slavonic oko, Lithuanian akis, Latin oculus, Greek okkos, Tocharian ak, ek, Armenian akn "eye."

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oeillade (n.)
"an oogling stare, an amorous gaze," 1590s, from French oeillide (15c.), from oeil "eye" (from Latin oculus, from PIE root *okw- "to see") + -ade.
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oculist (n.)

"eye doctor," 1610s, from French oculiste (16c.), from Latin oculus "an eye" (from PIE root *okw- "to see"). Middle English had oculister (early 15c.) "an authority on the eye and treatment of eye diseases."

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ocular (adj.)

c. 1500, "of or pertaining to the eye," from Late Latin ocularis "of the eyes," from Latin oculus "an eye," from PIE root *okw- "to see." As a noun, "eyepiece of an optical instrument," 1835, from the adjective.

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eyelet (n.)

"small hole," late 14c., oilet, from Old French oeillet, diminutive of oeil "eye," from Latin oculus "an eye" (from PIE root *okw- "to see"). Spelling later modified by influence of eye (n.).

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monocular (adj.)

"having only one eye; of or referring to vision with one eye," 1630s, from Late Latin monoculus "one-eyed," from Greek monos "alone, single" (from PIE root *men- (4) "small, isolated") + Latin oculus "eye" (from PIE root *okw- "to see").

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binocle (n.)
"telescope or opera glass with two tubes for use by both eyes at once," 1690s, from French binocle (17c.), from Latin bini- "two by two, twofold, two apiece" (see binary) + oculus "eye" (from PIE root *okw- "to see").
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binocular (adj.)
1738, "involving both eyes," earlier "having two eyes" (1713), from French binoculaire, from Latin bini "two by two, twofold, two apiece" (see binary) + ocularis "of the eye," from oculus "eye" (from PIE root *okw- "to see"). The double-tubed telescopic instrument (1871, short for binocular glass) earlier was called a binocle. Related: Binocularity; binocularly.
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ullage (n.)
"amount by which a cask or bottle falls short of being full," late 15c., from Anglo-French ulliage (early 14c.), Anglo-Latin oliagium (late 13c.), Old French ouillage, from ouiller "to fill up (a barrel) to the bung," literally "to fill to the eye," from ueil "eye" (perhaps used colloquially for "bung"), from Latin oculus (from PIE root *okw- "to see").
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