Plain English Meaning
Drugs that slow the brain down. They reduce pain and make a person drowsy or less alert, which is dangerous in the cockpit.
Full Definition
A class of drugs that depress the central nervous system, dulling the senses, slowing reaction time, and reducing alertness. In aviation medicine, the term covers prescription pain relievers (such as opioids), cough suppressants containing codeine, and similar substances that impair the brain's ability to respond quickly and accurately.
Context Anchor
Seen in aeromedical discussions about hypoxia, medication use, and whether a pilot is fit to fly.
Derivation
From the Greek 'narkotikos', meaning 'numbing' or 'making numb'. The original sense -- something that dulls sensation -- is exactly why these drugs are a flight safety concern.