A passing tone is a non-chord tone used to fill in the gap between two other tones. These other tones can belong to the same harmony, a different harmony, or may perhaps be non-chord tones as well.

Usually the space between the tones the passing tone separates is a some type of 3rd and the passing tone is given the interval which lies between; this rule is not without exception and especially during modulations one may find a passing tone which seperates two tones of no less than a second apart.