Official Aviation Definition
Bleed air is high-pressure, high-temperature air extracted from the compressor stage of gas turbine aircraft engines or the auxiliary power unit (APU). It is conditioned and routed throughout the airframe for cabin pressurization, air conditioning, wing anti-ice, and engine starting.
In Simple Words
Hot compressed air siphoned from the jet engines to heat the cabin, keep the plane pressurized, and melt ice off the wings.
Real-World Flight Example
The pneumatic isolation valve was closed to prevent hot bleed air from reaching the right engine anti-icing duct during the test.