domain (thing)

1. A discrete portion of a protein with its own function. The combination of domains in a single protein determines its overall function.
   
   
2. The highest level of biological classification, superseding kingdoms. The three domains of biological organisms are the Bacteria, the Archaea, and the Eukarya.

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