Categorical perception is a phenomenon of perception in which an individual assigns boundaries to stimuli varying along a continuum. In other words, the individual sorts continuous stimuli into stimulus categories. 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.