URL: 

1. n. [Formation Evaluation]

The exponent of porosity, m, in the relation of formation factor, F, to porosity, phi. In the Archie equation, F = 1 / phim, H. Guyod termed m the cementation exponent because m was observed to be higher in cemented rock. The more general term is porosity exponent.

Reference:
Guyod H: Fundamental Data for the Interpretation of Electric Logs, The Oil Weekly 115, no. 38 (October 30, 1944): 21-27.