1. a drug of abuse (e.g., alcohol, cannabis, cocaine, an inhalant), a medication (e.g., a sedative or anxiolytic), or a toxin that is capable of producing harmful effects when ingested or otherwise taken into the body. See substance-related disorder.

2. in philosophy, that which has an independent, self-sufficient existence and remains unalterably itself even though its attributes or properties may change. Philosophers have differed over what qualifies as a substance and whether reality consists of a single substance (see monism) or more (see dualism; pluralism).