Plain English Meaning
Your brain shows you things that aren't real, or twists what is real, so you can't trust what you're sensing.
Full Definition
Sensory experiences in which a pilot sees, hears, or feels things that are not actually present, or perceives real things in a distorted way, caused by certain drugs, medications, fatigue, hypoxia, or illness. These false perceptions can affect vision, hearing, balance, and judgment, and may persist for hours after the original cause has passed.
Context Anchor
Seen in aeromedical and drug-safety discussions, especially when explaining why certain drugs are unsafe before or during flight.
Derivation
From the Latin hallucinari, meaning to wander in the mind or talk wildly. The original sense was a mind drifting away from reality, which is exactly what happens when senses report something that isn't there.