c. 1600, "frightening ghost," from French spectre "an image, figure, ghost" (16c.), from Latin spectrum "appearance, vision, apparition" (see spectrum). Figurative sense "object of dread" is from 1774.
c. 1600, in ancient Greek music, in reference to one of the three standard tetrachords, from French diatonique, from Latin diatonicus, from Greek diatonikos, from diatonos "extending; pertaining to the diatonic scale," from dia "through, across" (see dia-) + teinein "to stretch," from PIE root *ten- "to stretch." In modern music, "using tones, intervals, and harmonies of the standard major and minor scales without chromatic alteration," 1690s. Related: Diatonically.
early 15c., "pertaining to the faculty of sight;" also "coming from the eye or sight" (as a beam of light was thought to do), from Late Latin visualis "of sight," from Latin visus "a sight, a looking; power of sight; things seen, appearance," from visus, past participle of videre "to see" (see vision). Meaning "perceptible by sight" is from late 15c; sense of "relating to vision" is first attested c. 1600. The noun meaning "photographic film or other visual display" is first recorded 1944.
late 14c., optik, "of or pertaining to the eye as the organ of vision," from Old French optique, obtique (c. 1300) and directly from Medieval Latin opticus "of sight or seeing," from Greek optikos "of or having to do with sight," from optos "seen, visible," related to ōps "eye," from PIE root *okw- "to see." Meaning "relating to or pertaining to vision or sight" is from 1590s. Optics "eyes" is from 1640s; "formerly the learned and elegant term; afterwards pedantic, and now usually humorous" [OED].
"to dim (the vision) with tears, rheum, etc.," also "to have watery or rheumy eyes," early 14c., of uncertain origin, possibly from an unrecorded Old English *blerian, which is perhaps related to blur. Related: Bleared; blearing.
c. 1300, blere, of the eyes, "watery, rheumy, sore or dimmed with watery discharge," from or related to blear (v.). Compare Middle High German blerre "having blurred vision," Low German bleeroged "blear-eyed."