Plain English Meaning
Something that slows down how your brain and nerves work, making you think and react more slowly.
Full Definition
A substance that slows the activity of the brain and spinal cord, reducing alertness, reaction time, judgment, and coordination. Alcohol is the most common example; many sedatives, tranquilizers, and certain prescription and over-the-counter medications also fall into this category.
Context Anchor
Seen in aviation medical and alcohol discussions, especially when explaining why alcohol affects pilot performance even when a person does not feel drunk.
Derivation
From Latin deprimere, meaning 'to press down.' A depressant 'presses down' the activity of the nervous system — slowing it rather than stimulating it. This helps why these substances reduce, rather than improve, performance.