Any of several techniques to reduce the chances that an enzyme will inactivate in vitro (see enzyme inactivation) by changing the environmental conditions (such as pH, temperature, concentration of salt, etc.) or by attaching organic groups to it or changing some of its amino acid subunits.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.