Datum is the Latin singular of data, and its survival in English is a curiosity. The word is the neuter past participle of Latin dare, "to give" — a datum is literally "a thing given," something granted at the outset of an argument for the sake of reasoning from it.
English keeps a small set of these unnaturalised Latin nouns with their original endings intact: alumnus and alumni, index and indices, formula and formulae. Datum belongs to the set on paper but has quietly lost the fight in practice. Almost nobody says datum any more, and data — grammatically a plural — is now treated by most English speakers as a mass noun taking a singular verb. That is the normal fate of a borrowed inflection: it survives as long as the borrowers know the source language, and no longer.
datum (n.) proper Latin singular of data (q.v.).