An Etymological Dictionary of Astronomy and Astrophysics

The invisible part of electromagnetic spectrum possessing wavelengths between those of visible light and those of radio waves, i.e. approximately between about 0.75 and 1000 μm.

Etymology (EN): From L. infra (adv., prep.) “below, underneath, beneath,” inferus “lower,” from PIE *ndher; cf. Av. aδara- (adj.), aδairi- (prep.) “below;” Mid.Pers. êr “below, down; low, under,” adar “low,” azêr “below, under” (Mod.Pers. zir “below, down”); Skt. ádhara- “lower;” O.E. under “under, among”); → red.

Etymology (PE): Forusorx, from foru “down, downward; below; beneath;” Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain,” + sorx→ red.