Galdr (plural galdrar) is one Old Norse word for “spell, incantation”; these were usually performed in combination with certain rites.[1] It was mastered by both women and men.[2] Some scholars have assumed they chanted it in falsetto (gala).[2][3]

[…] The Old English forms were gealdor, galdor, ȝaldre “spell, enchantment, witchcraft”, and the verb galan meant “sing, chant”. It is contained in nightingale (from næcti-galæ), related to giellan, the verb ancestral to Modern English yell; compare also the Icelandic verb að gala “to sing, call out, yell” and Dutch gillen “to yell, scream”.

[…] It is also mentioned in several of the poems in the Poetic Edda, and for instance in Hávamál, where Odin claims to know 18 galdrar.[1] For instance, Odin mastered galdrar against fire, sword edges, arrows, fetters and storms, and he could conjure up the dead and speak to them.[8][9]

Source: Galdr – Wikipedia