Heterophoria is a generic term for a latent tendency to imbalance of the ocular axes (latent strabismus; cf. heterotropia). This may be clinically demonstrated using the cover-uncover test: if there is movement of the covered eye as it is uncovered and takes up fixation, this reflects a phoria. Phorias may be in the horizontal (esophoria, exophoria) or vertical plane (hyperphoria, hypophoria).
Shaunak S, O’Sullivan E, Kennard C. Eye movements. In: Hughes RAC (ed.). Neurological investigations. London: BMJ Publishing, 1997: 253-282