spectrum.

From spec(iō) (“look at, behold”) + -trum (making it a doublet of speculum). The only attestation in Classical antiquity is in a pair of letters between Cicero and Cassius Longinus which imply that the Epicurean Catius (fl. c. 50s–40s BC) used spectrum as a translation of the Greek philosophical term εἴδωλον (eídōlon, “image”). It may therefore have been coined by Catius as a neologism, although alternatively, it could be an undocumented but preexisting word that he repurposed as a technical term. After Cicero, the word is extremely sparsely attested until being revived around the start of the sixteenth century by Renaissance humanist authors with the meaning "apparition" or "phantom", possibly influenced by the fact that Greek εἴδωλον also had this sense. The scientific use to refer to the visible spectrum of colored light was first introduced by Isaac Newton, who used the word in the second half of the seventeenth century in both his English writings and in his first Latin draft of the Opticks, the Fundamentum Opticae, although the 1706 Latin translation of Opticks by Samuel Clarke translates Newton's English spectrum into Latin as imago.