Plain English Meaning
The angled line through the air that an airplane flies down along on its way to landing. It is a steady, gentle descent aimed at the runway.
Full Definition
The descending flight path an aircraft follows from the final approach point to the runway touchdown zone, typically defined by an electronic signal (such as the glideslope of an ILS) or by visual aids (such as a VASI or PAPI). A standard glidepath is approximately 3 degrees below horizontal.
Context Anchor
You will see this term in approach and landing discussions, including how can affect the last part of the landing.