What does 'grace note' refer to in music?

The term "grace note" in music refers to a quick note that is played just before a main note, providing embellishment and enhancing the expressiveness of the melody. These notes are typically shorter in duration than the notes they precede and are often not counted in the rhythmic structure of the music. Their purpose is to add flair and articulation, making the main note more interesting.

In this context, the other options do not accurately capture what a grace note is. The idea of a main note in a melody fails to acknowledge the specific function grace notes serve as embellishments rather than principal melodic components. The notion of a note that is prolonged is more descriptive of other musical techniques such as ties or fermatas, whereas grace notes are inherently quick. Lastly, a note that adds harmony would refer to chord tones or intervals, which is not the role of grace notes, as they do not serve a harmonic function but rather an ornamental one.