Alkaloid, any of a class of naturally occurring organic nitrogen-containing bases. Alkaloids have diverseand important physiological effects on humans and other animals. Well-known alkaloids include morphine, strychnine, quinine, andnicotine any of various solid or semisolid amorphous fusible flammable natural organic substances that are usually transparent or translucent and yellowish to brown, are formed especially in plant secretions, are soluble in organic solvents (such as ether) but not in water, are electrical nonconductors, and are used chiefly in varnishes, printing inks, plastics, and sizes and in medicine