<. As an adjective by 1859.
- Any of various organic compounds, such as nicotine and morphine, that have basic chemical properties and that usually contain at least one nitrogen atom in a heterocyclic ring.
- Any of a large class of naturally occurring, complex organic compounds that contain nitrogen and have physiological effects on animals, including humans. Most alkaloids occur in plants, although some are produced by fungi and animals. Alkaloids are bases and usually form colorless crystalline solids with a bitter taste. They have a wide range of effects and are used as medicines and poisons. Morphine, quinine, strychnine, codeine, caffeine, cocaine, and nicotine are all alkaloids.