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any of a large class of organic, nitrogen-containing ring compounds of vegetable origin and sometimes synthesized, some of which are liquid but most of which are solid, that have a bitter taste, that are usually water-insoluble and alcohol-soluble, that combine with acids without the loss of a water molecule to form water-soluble hydrochlorides, hydrobromides, or the like, and that usually exhibit pharmacological action, as nicotine, morphine, or quinine.
resembling an ; alkaline.
Historical Examples

As usually taken, of course, the proportion of the alkaloid is much less.
The Natural Cure of Consumption, Constipation, Bright’s Disease, Neuralgia, Rheumatism, Charles Edward Page

Mr. alkaloid had come from Ryde to London to get his hair singed.
Punch, or the London Charivari, Vol. 98 February 15, 1890 Various

Tea and coffee both contain the alkaloid caffeine, whilst cocoa contains theobromine.
Encyclopaedia Britannica, 11th Edition, Volume 6, Slice 6 Various

Within the room Mr. alkaloid was photographing the dead poodle.
Punch, or the London Charivari, Vol. 98 February 15, 1890 Various

Containing but traces of the alkaloid they are not bitter, and possess a pleasant, but very volatile aroma.
Coca and its Therapeutic Application, Third Edition Angelo Mariani

It was but for a moment, for the action of the alkaloid is rapid.
A Study In Scarlet Arthur Conan Doyle

From it an alkaloid is extracted that is called by Manila chemists, ditaïne.
The Philippine Islands Ramon Reyes Lala

Picrotoxin (C30H24O13) is not an alkaloid as may be seen from its formula.
The Medicinal Plants of the Philippines T. H. Pardo de Tavera

Of all salts this is most suitable for subcutaneous injection; it is the form in which the alkaloid exists in opium.
Poisons: Their Effects and Detection Alexander Wynter Blyth

The oil of hyoscyamus is poisonous, and contains the alkaloid.
Poisons: Their Effects and Detection Alexander Wynter Blyth

noun
any of a group of nitrogenous basic compounds found in plants, typically insoluble in water and physiologically active. Common examples are morphine, strychnine, quinine, nicotine, and caffeine
n.

1831, from alkali (q.v.) + -oid. “A general term applied to basic compounds of vegetable origin, bitter in taste, and having powerful effects on the animal system” [Flood]. As an adjective by 1859.

alkaloid al·ka·loid (āl’kə-loid’)
n.
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.
alkaloid
(āl’kə-loid’)
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.