Galdr (plural galdrar) is one Old Norse word for “spell, incantation”; these were usually performed in combination with certain rites. It was mastered by both women and men. Some scholars have assumed they chanted it in falsetto (gala).
[…] 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. For instance, Odin mastered galdrar against fire, sword edges, arrows, fetters and storms, and he could conjure up the dead and speak to them.
Source: Galdr – Wikipedia