Definition
The specified path that aircraft are required to fly while operating in the vicinity of an aerodrome, typically consisting of a rectangular pattern of upwind, crosswind, downwind, base, and final legs flown at a defined altitude.
Plain English
It's the standard rectangular flight path pilots fly around an airport when taking off, landing, or staying in the local area, so that everyone follows the same predictable route.
Context Anchor
You encounter this term in procedures for operating near an aerodrome, especially when entering, flying around, or leaving the area used by aircraft that are landing or taking off.
Derivation
Aerodrome' comes from Greek 'aēr' (air) and 'dromos' (course or running track). 'Circuit' comes from Latin 'circuitus' meaning 'a going around.' So the term literally describes a running track in the air around an airport, which is exactly what the pattern looks like from above.
Why Pilots Care
Following the circuit keeps traffic flowing safely, reduces collision risk, and allows pilots to see and be seen by others in the pattern.
Intuition Check
Circuit does not mean an electrical circuit here, and traffic does not mean road traffic. In this context, it means the planned route aircraft follow near an aerodrome so their movement is predictable.
Example Sentence 1
After takeoff, the student climbed straight ahead before turning crosswind to join the aerodrome traffic circuit.
Example Sentence 2
When the tower clears you to land you may leave the aerodrome traffic circuit and turn onto final approach.