āter- / Indo-European roots
1. Suffixed zero-grade form *ātr-o-. atrabilious from Latin āter (feminine ātra), black (< "blackened by fire").
2. Suffixed zero-grade form *ātr-yo-. atrium from Latin ātrium, forecourt, hall, atrium (perhaps originally the place where the smoke from the hearth escaped through a hole in the roof).
3. Compound shortened zero-grade form *atro-əkw-, "black-looking" (*əkw-, "looking"; see okw-) atrocious from Latin ā̆trōx, frightful.
4. Basic form *āter. zircon from Old Persian *ātar, fire (stem āç- attested in month name āçiyādiya, "(month) of fire-worship"), from Indo-Iranian *ātar.
5. Possibly, but obscurely related to this root is Sanskrit atharvā, atharvan-, priest (-van-, possessive suffix): Atharva-Veda
[Pokorny āt(e)r- 69.]