Depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object. Depth sensation is the ability to move accurately, or to respond consistently, based on the distances of objects in an environment.^{[citation needed]}

Depth perception arises from a variety of depth cues. These are typically classified into binocular cues that require input from both eyes and monocular cues that require the input from just one eye. Binocular cues include stereopsis, yielding depth from binocular vision through exploitation of parallax. Monocular cues include size: distant objects subtend smaller visual angles than near objects.

Monocular cues provide depth information when viewing a scene with one eye.

Binocular cues provide depth information when viewing a scene with both eyes.

Of these various cues, only convergence, accommodation and familiar size provide absolute distance information. All other cues are relative (i.e., they can only be used to tell which objects are closer relative to others). Stereopsis is merely relative because a greater or lesser disparity for nearby objects could either mean that those objects differ more or less substantially in relative depth or that the foveated object is nearer or further away (the further away a scene is, the smaller is the retinal disparity indicating the same depth difference).