Plain English Meaning
It is the way a Terminal Arrival Area is shaped and laid out on an RNAV approach chart. The space around the approach is divided into sectors, each with its own safe altitude, so a pilot arriving from any direction has a clear, pre-planned way to join the approach.
Full Definition
The standardized layout of a Terminal Arrival Area associated with an RNAV (GPS) instrument approach, consisting of defined areas around the initial approach fixes that provide minimum safe altitudes and a structured path from the en route environment onto the approach. The basic TAA design uses three areas — a and two areas ( and ) — arranged in a 'T' around the , each bounded by radii and bearings from the approach fixes, with a published that guarantees within that .