Source : Webster's Revised Unabridged Dictionary (1913)

Alkaloid \Al"ka*loid\ ([a^]l"k[.a]*loid), n. (Chem.)
   An organic base, especially one of a class of substances
   occurring ready formed in the tissues of plants and the
   bodies of animals.

   Note: Alkaloids all contain nitrogen, carbon, and hydrogen,
         and many of them also contain oxygen. They include many
         of the active principles in plants; thus, morphine and
         narcotine are alkaloids found in opium.

Alkaloid \Al"ka*loid\ ([a^]l"k[.a]*loid), Alkaloidal
\Al`ka*loid"al\ ([a^]l`k[.a]*loid"al), a. [Alkali + -oid: cf. F.
   alcalo["i]de.]
   Pertaining to, resembling, or containing, alkali.

Source : WordNet®

alkaloid
     n : natural nitrogen-containing bases found in plants