Phorias are known as 'latent squint' because the tendency of the eyes to deviate is kept latent (hidden) by fusion.
[1] A person with two normal eyes has single vision (usually) because of the combined use of the sensory and motor systems.
The opposite of heterophoria, where the eyes are straight when relaxed and not fusing, is called orthophoria.
In contrast, fixation disparity is a very small deviation of the pointing directions of the eyes that is present while performing binocular fusion.
When the fusional vergence system can no longer hold back heterophoria, the phoria manifests.
Heterophoria is the misalignment of the visual axis such that one or both eyes are not properly fixated to an object of interest.