A variate occurring concomitantly with the variate of primary interest and measured for the purpose of making informed adjustments on the variate of primary interest (e.g., measuring pretest performance of two groups in order to adjust their post-test scores so that they take account of differences between groups that existed prior to the treatment of one of the groups).
Yarbrough, D. B., Shulha, L. M., Hopson, R. K., and Caruthers, F. A. (2011). The program evaluation standards: A guide for evaluators and evaluation users (3rd ed.). Thousand Oaks, CA: Sage.