Noun
In medieval music, a mnemonic for the Latin words saeculōrum and āmēn (from “ […] in saecula saeculōrum. Āmēn.” from the Gloria Patri doxology), used in liturgical works to indicate how the words should be sung with various cadences.
(by extension) A cadence used to sing those words of the Gloria Patri.
Source: en.wiktionary.org