What term refers to the enlargement of note values at a constant ratio?

The term that refers to the enlargement of note values at a constant ratio is called augmentation. In music, augmentation specifically involves increasing the duration of notes or rhythmic values, typically by a fixed factor. For example, if a piece written in quarter notes is augmented, the quarter notes might be transformed into half notes, thereby doubling their duration. This technique can add a different flavor and texture to music, allowing composers and performers to explore variations in tempo and rhythm while maintaining the piece's overall structure and integrity.

In contrast, the other terms do not capture this concept. Diminuendo relates to a gradual decrease in volume, while extension refers to expanding musical phrases but does not intrinsically involve altering note values. Acceleration denotes a speeding up of tempo rather than an enlargement of rhythmic values. Therefore, augmentation is the correct term to describe the enlargement of note values at a constant ratio.