Plain English Meaning
Common allergy and cold medicines (such as those found in many over-the-counter products) that relieve sneezing, runny nose, and itching, but often make you sleepy and slow your reactions. Pilots are generally not allowed to fly while taking them.
Full Definition
A class of medication used to treat allergy symptoms and the common cold by blocking the action of histamine, a chemical the body releases during an allergic reaction. Many antihistamines cause drowsiness and slowed reaction times, and most are disqualifying for flight duties.
Context Anchor
Seen in aeromedical and preflight decision-making discussions about drugs, over-the-counter medicines, and whether a pilot is fit to fly.
Derivation
From 'anti-' (against) and 'histamine' (the body chemical that triggers allergy symptoms). The name simply means 'against histamine' — the drug works by blocking histamine's effects.