An approach note is usually a quarter note or less in duration which moves by step to a chord tone or available tension.
Approach notes are found in many configurations:
1. Passing tones: These are approach notes that connect two notes of different pitch in stepwise motion.
2. Auxiliary tones (also called neighbor tones): An auxiliary tone moves away from and returns to the same pitch by stepwise motion.
3. Unprepared approach notes: These are approach notes that are preceded by a leap, a rest, or the same pitch.
"Modern Jazz Voicings: Arranging for Small and Medium Ensembles" by Ted Pease and Ken Pullig, Berklee Press, Boston, 2001, page 19