A central nervous system depressant reduces the activity of the brain and spinal cord, producing drowsiness, slowed reactions, reduced coordination, and at higher exposures slowed breathing. Alcohol, opioids, benzodiazepines, some muscle relaxants, sedating antihistamines, and certain seizure and sleep medicines all belong in this broad group.
The practical importance is additive effect. Two mild depressants taken together can produce impairment that neither would cause alone, which is why official labeling and pharmacists ask about every sedating product a person uses, including over-the-counter sleep aids and alcohol.