1. The process whereby a gas becomes a liquid or a solid.
  2. A chemical reaction between two organic compounds which produces (among other things) water, ammonia, or a simple alcohol.
  3. A chemical reaction between two molecules which links them together and expels a molecule of water. For example, the joining of two amino acids by a peptide bond during the formation of a polypeptide.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.