‘Commencement of flight’ is the point when an aircraft begins to move under its own power for the purpose of preparing for take-off.
CS-MMEL (Certification Specifications and Guidance Material for Master Minimum Equipment List), Book 2 – Guidance Material, Subpart B, Master Minimum Equipment List, GM5 MMEL.120 Format and content of MMEL, Definitions And Explanatory Notes
For an older reference to JAR:
Commencement of flight is defined in JAR-MMEL/MEL.001(d) and .005(d) as “the point when an aircraft begins to move under its own power for the purpose of preparing for take-off.” In the case of a helicopter, it refers to the moment the helicopter commences air or ground taxi.