Final answer:
A piece of music that starts on the last beat of a measure begins with an anacrusis or a pickup, terms that are synonymous for an incomplete measure that leads into the first full measure.
Step-by-step explanation:
When a piece of music begins on the last beat of a measure, it is said to begin with an anacrusis or a pickup. These terms are often used interchangeably in music theory to describe an incomplete measure at the beginning of a piece that leads into the first full measure. It sets the momentum for the melody and often creates a sense of forward motion. An anacrusis is not to be confused with a fermata, which is a symbol that indicates a note or rest should be held longer than its original value, or a ritardando, which is a gradual decrease in tempo.