Euouae is a mnemonic

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 which was used in medieval music

Music

Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...

 to denote the sequence of tones in the "seculorum Amen" passage of the lesser doxology

Doxology

A doxology is a short hymn of praises to God in various Christian worship services, often added to the end of canticles, psalms, and hymns...

, Gloria Patri, which ends with the phrase In saecula saeculorum

In saecula saeculorum

The Latin phrase expresses the idea of eternity. It is biblical, taken from the Vulgate translation of the New Testament, rendering Greek . The usual English translation is "for ever and ever", but in Ephesians 3:21, the KJV notably has "world without end"...

, Amen

Amen

The word amen is a declaration of affirmation found in the Hebrew Bible and New Testament. Its use in Judaism dates back to its earliest texts. It has been generally adopted in Christian worship as a concluding word for prayers and hymns. In Islam, it is the standard ending to Dua and the...

. In plainchant sources, the differentia, that is, the melodic formula to be sung at the end of every line of chanted psalmody, would be written over either the letters EUOUAE, or merely E----E, representing the first and last vowel of "seculorum Amen."

According to the Guinness Book of World Records, at six letters long, is the longest word in the English language

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 which is made up of nothing but vowel

Vowel

In phonetics, a vowel is a sound in spoken language, such as English ah! or oh! , pronounced with an open vocal tract so that there is no build-up of air pressure at any point above the glottis. This contrasts with consonants, such as English sh! , where there is a constriction or closure at some...

s; it is also the English word with the most consecutive vowels.