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."

1560s, "relating to or connected with the science of optics; pertaining to vision," from optic + -al (1). Of abstract art, from 1964. In astronomy, in reference to double stars that appear so only because they lie in the same line of sight from earth, by 1868. Optical illusion is attested by 1757. Related: Optically.

1680s, "person skilled in the science of optics, a sense now rare or obsolete; see optic + -ian. In the meaning "one who makes or sells optical instruments" it is attested from 1737, after French opticien "maker or seller of optical instruments," on the model of physician, etc.

"science of sight and the natural properties of light," 1570s, from optic; also see -ics. Used for Medieval Latin optica (neuter plural), from Greek ta optika "optical matters," neuter plural of optikos "optic."