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."

"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."

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.

"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.).

"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").