Specific activity is a term used in measuring enzyme kinetics (rate of reaction of an enzyme with a particular substrate). This term is defined as the amount of substrate the enzyme converts (reactions catalyzed), per mg protein in the enzyme preparation, per unit of time.
Specific activity is a measure of enzyme purity. The value becomes larger as an enzyme preparation becomes more pure, since the amount of protein (mg) is typically less, but the rate of reaction stays the same (or may increase due to reduced interference or removal of inhibitors).
Source:
Nelson, D. and Cox, M. 2000. Lehninger Principles of Biochemistry, 3rd Edition. Worth Publishers, New York, NY, USA.
The specific activity of the isolated enzyme was measured at 150 umoles/min/mg protein before purification and 800 umoles/min/mg, after purification.