Which ornament is known for being a grace note?

The ornament known as a grace note is the acciaccatura. An acciaccatura is a very short, quick note that is played right before a main note and is typically not counted in the rhythm of the measure. It adds embellishment and expressiveness to the melodic line, allowing the performer to create a more intricate musical phrase.

In this context, the acciaccatura is characterized by its rapid execution, often notated with a small, slashed notehead, indicating its role as a decorative element rather than a primary note. This distinguishes it from the other options, which serve different functions in music. The mordent involves a rapid alternation between a note and the note immediately above or below it. A turn consists of a series of notes that outline a specific pattern, while a trill involves the rapid alternation between a given note and its immediate neighbor, typically lasting for an extended duration.