Official Aviation Definition
In an airfield traffic pattern, the downwind leg is a course flown parallel to the runway in the direction opposite to landing. Pilots fly this leg at standard pattern altitude (typically 1,000 feet AGL) while configuring the aircraft for landing.
In Simple Words
Flying alongside the runway in the opposite direction before turning around to line up and land.
Real-World Flight Example
Abeam the runway threshold on the downwind leg, the pilot lowered the landing gear and extended the first notch of flaps.