Plain English Meaning
It happens when exhaust fumes leak into the cabin and the pilot breathes in a poisonous gas that stops the blood from carrying oxygen. The pilot may feel sleepy, dizzy, or get a headache without realizing why, and can pass out if they don't get fresh air.
Full Definition
A condition in which carbon monoxide (CO) — a colorless, odorless, tasteless gas produced by incomplete combustion of fuel — is inhaled and binds to hemoglobin in the blood, blocking oxygen from being carried to the body's tissues. In aviation, it most commonly results from exhaust gases leaking into the cabin through a cracked or corroded exhaust shroud used by the cabin heating . Symptoms include headache, , dizziness, blurred vision, and , progressing to unconsciousness and death if continues.