**Analysis of Covariance
(ANCOVA)**: A General Linear Model containing one or more
covariates, usually in addition
to one or more categorical factors. Each covariate X is tested for a linear
trend with the continuous response Y. Transformations of response or covariate
may be necessary to achieve linearity prior to analysis.

A covariate factor X 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.

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/