The Collaborative International Dictionary of English v.0.48:

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.]
   1. Pertaining to, resembling, or containing, alkali.
      [1913 Webster]

   2. of or pertaining to alkaloids.
      [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Alkaloid \Al"ka*loid\ ([a^]l"k[.a]*loid), n. (Chem.)
   An organic base, especially one of a class of
   nitrogen-containing substances occurring ready formed in the
   tissues of plants and the bodies of animals.
   [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

alkaloid
    n 1: natural bases containing nitrogen found in plants