The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
ter-
DEFINITION:
Fire. 1. Suffixed zero-grade form *tr-o-.atrabilious, from Latin ter (feminine tra), black (< blackened by fire). 2. Suffixed zero-grade form *tr-yo-.atrium, from Latin trium, 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-kw-, black-looking (*kw-, looking; see okw-). atrocious, from Latin trx, frightful. 4. Basic form *ter.zircon, from Old Persian *tar, fire (stem ç- attested in month name çiydiya, (month) of fire-worship), from Indo-Iranian *tar.5. Possibly, but obscurely related to this root is Sanskrit atharv, atharvan-, priest (-van-, possessive suffix): Atharva-Veda. (Pokorny t(e)r- 69.)