Covariate: A factor X that varies at least on an ordinal scale, and usually on a continuous scale (such as time, distance, etc) and is therefore a covariate of the response Y. Analysis of covariance assumes that the response has a linear relation to the covariate, and transformations of response or covariate may be necessary to achieve this prior to analysis.

Doncaster, C. P. & Davey, A. J. H. (2007) Analysis of Variance and Covariance: How to Choose and Construct Models for the Life Sciences. Cambridge: Cambridge University Press.

http://www.southampton.ac.uk/~cpd/anovas/datasets/