The Collaborative International Dictionary of English v.0.48:

Passing \Pass"ing\, a.
   1. Relating to the act of passing or going; going by, beyond,
      through, or away; departing.
      [1913 Webster]

   2. Exceeding; surpassing, eminent. --Chaucer. "Her passing
      deformity." --Shak.
      [1913 Webster]

   Passing note (Mus.), a character including a passing tone.


   Passing tone (Mus.), a tone introduced between two other
      tones, on an unaccented portion of a measure, for the sake
      of smoother melody, but forming no essential part of the
      harmony.
      [1913 Webster]

WordNet (r) 3.0 (2006):

passing note
    n 1: a nonharmonic note inserted for transition between harmonic
         notes [syn: passing note, passing tone]