Uncertain. Commonly considered a substratum word. Probably ultimately from Proto-Indo-European *h₁eh₂ter-, *eh₂ter- through some intermediate; compare Sanskrit अथर्वन् (atharvan, “fire”), Avestan 𐬁𐬙𐬀𐬭𐬱 (ātarš, “fire”), Albanian vatër (which may come ultimately from an Iranic language such as Scythian or Sarmatian). Sometimes considered derived from the Albanian; cf. also Serbo-Croatian vatra (“fire”). The relationships between the languages are unclear, and it is disputed which language borrowed from which, or if it had a common Balkanic source. It may be that some of the Slavic cognates were borrowed from Romanian or other Vlach languages. Another theory suggested is Byzantine Greek βάθρον.