Plain English Meaning
Every part of a flight, from getting ready on the ground to shutting down at the gate. If something is required for all phases of flight, it has to be in place the whole time, not just in the air.
Full Definition
A regulatory and operational expression covering every segment of a flight from preflight planning through engine shutdown, including taxi, takeoff, climb, cruise, descent, approach, landing, and post-landing rollout. When a rule, procedure, or piece of equipment is described as applying to all phases of flight, it applies continuously across every one of these segments without exception.