Meaning

originFrom the vowels in the last two words of the final phrase of the Gloria Patri doxology in Latin: “[…] in saecula saeculōrum. Āmēn.” (literally “[…] into ages of ages (for ever and ever). Amen.”). In liturgical works used in Gregorian chant, instead of repeating the words, the letters were printed under musical notes to indicate how the syllables of the words should be sung in different melodies.

Etymology from Wiktionary, CC BY-SA 3.0