The Avestan word ātar-
Word ātar-
Meaning fire
See also Homeric Greek: αἶθος ‘a burning heat, fire’
Proto-Indo-European: *h₂éhₓtr̥ ‘fire’
Proto-Celtic: *aidu ‘fire’

Other words that might be of interest
təmah- ‘darkness’, hvarə ‘sun’, axšaēna ‘black, dark’, raocah- ‘light’, huuar- ‘sun’