What is the term used when a piece of music in a minor key ends with a major chord?

The correct term for a piece of music in a minor key that concludes with a major chord is known as a Tierce de Picardie. This practice involves resolving a piece that is primarily in a minor tonality with a major chord, specifically the tonic chord of the key, resulting in a bright or uplifting conclusion. The effect offers contrast and can evoke a sense of surprise or resolution, thereby creating an interesting musical departure from the preceding minor atmosphere.

In this context, the other terms do not accurately describe this specific musical phenomenon. While a cadence refers to a progression of chords that concludes a phrase, it does not specify the minor to major transition characteristic of the Tierce de Picardie. An augmented chord involves the interval of an augmented fifth, which is unrelated to the resolution of minor to major. A plagal cadence typically involves a specific progression (IV to I) and doesn't specifically denote the transition from minor to a major tonic chord. Thus, the Tierce de Picardie clearly identifies this unique compositional technique.