Interpolated chord is that one which appears in the middle of a “harmonic cliché”. For example, in the cliché II – V – I:
| Dm7 | G7 | C |
If we put the chord Ab7 before G7, it would be considered as an interpolated chord:
| Dm7 | Ab7 G7 | C |
Notice that Ab7 is acting as subV7.
Learn more here: http://www.simplifyingtheory.com/interpolated-chord/