Categorical perception is a phenomenon of perception in which an individual assigns boundaries to stimuli varying along a continuum. Categorical perception was first observed for auditory stimuli (speech sounds) but has since been found to be applicable to other perceptual modalities.

Source:  Fugate, Jennifer M. B. (2013). Categorical Perception for Emotional Faces. Emotion Review. 2013 Jan; 5(1): 84–89.