Any of a class of over 3000 known nitrogen-containing compounds (such as morphine, caffeine, cocaine and nicotine) that are typically basic (pH over 7) and are produced by plants but have strong physiological effects on animals (including humans). They are synthesized from amino acid precursors such as tryptophan and tyrosine.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.