spectrum

English spectrum comes from Latin spectrum, meaning "appearance, image, apparition," itself from the verb speciō, "to look at, view." That verb traces back through Proto-Italic *spekjō to the Proto-Indo-European root *speḱ-, "to see or watch."

Latin speciō, "to look at," was joined with the suffix -trum to form spectrum, an "appearance" or even "apparition" — something seen. English borrowed the word directly from Latin with that same sense of a visible image.

Spectrum is a doublet of specter, meaning both words descend from the same Latin spectrum but reached English by different paths. Speciō, the root verb meaning "to look at," is also related to the English word scope.