What does 'dim.' indicate in a musical score?

The notation 'dim.' in a musical score stands for "diminuendo," which instructs musicians to gradually decrease the volume of the music. This creates a fading effect, allowing for emotional nuance and expression in the performance. It is commonly used to guide performers in shaping the dynamics of a piece, often enhancing the overall musicality by creating contrast within the dynamics.

The other options describe different musical terms: "to grow louder" would be indicated by "crescendo," "to play quickly" refers to a tempo instruction such as "allegro" or "presto," and "to return to the main theme" aligns with the term "da capo." Each of these terms carries a distinct meaning, separate from the concept of diminishing sound represented by 'dim.'