What is the term for the cadence known as the "amen cadence"?

The term for the cadence known as the "amen cadence" is indeed the plagal cadence. This particular cadence traditionally follows a progression from the subdominant chord to the tonic chord, often creating a sense of resolution that is particularly associated with the phrase "amen," typically sung at the end of hymns and religious pieces.

In the context of choral and sacred music, the plagal cadence provides a soothing and conclusive feel, which resonates well in liturgical settings where this phrase is often used. This quality of resolution and the connection to worship music highlights its significance in music theory and practice.

Other cadences, such as the authentic cadence—which moves from the dominant to the tonic—establish a strong sense of finality or closure, but do not carry the same association with the "amen" at the end of a piece. The deceptive cadence shifts expectation by moving to a chord other than the tonic after building up to what seems to be a conclusive moment, and the half cadence leaves the listener expecting resolution without providing it. Therefore, the plagal cadence uniquely fulfills this role tied to the traditional usage of the "amen" phrase in music.