Apparently a primary word; a "mother" (literally or figuratively, immediate or remote) -- mother.

μητέρα (mētera) − 26 Occurrences

μητέρας (mēteras) − 2 Occurrences

μήτηρ (mētēr) − 32 Occurrences

μητρὶ (mētri) − 11 Occurrences

μητρὸς (mētros) − 12 Occurrences