grace note

Music a note not necessary to the melody, added only for ornamentation: it is usually printed as a small note with a slant line through the stem, just before the note that it embellishes, its short time value subtracted from the preceding or following note, depending on context

See grace note in American Heritage Dictionary 4

noun
A musical note, especially an appoggiatura, that is added as an embellishment, and is printed in small type and not counted in rhythm.
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