**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/