Lewis & Short

spectrum, i, n. [specio], an appearance, form, image of a thing; an apparition, spectre, the Gr. εἴδωλον (much less freq. than simulacrum and species): spectra Catiana … nam quae Democritus εἴδωλα, Catius Insuber Epicureus spectra nominat, Cic. Fam. 15, 16, 1 sq.: spectra Catiana, Cass. ap. Cic. Fam. 15, 19, 1.