Attraction between particles or substances to other particles or substances. In biological and biochemical fields, this term generally is a measure of the attraction of one biological molecule toward another molecule (which can be organic or inorganic), either to alter it, destroy it, or form a compound with it. Examples are enzymes and their substrates, or antibodies and their antigens.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.