Accidental note is any note that does not belong to a certain scale or tonality. For example, see the C major scale:
C, D, E, F, G, A, B
The notes C#, D#, F#, G#, A# are called accidentals, in this case, because they don’t belong to this scale.
Learn more here: http://www.simplifyingtheory.com/accidental-notes/