Definition

The flight plan, including changes, if any, brought about by subsequent clearances.

Source: ICAO Doc 4444 PANS-ATM

Description

Simply put, CPL = FPL + ATC. The flight plan (FPL) expresses the intent of how the flight would be performed. It can be amended by change messages (CHG) which are filed by e.g. the pilot or another authorized person. The CHG messages replace or add data in various parts of the original FPL. 

In most cases the flight follows the CPL rather than the FPL that was filed before departure. The reasons for this is that often either: