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.