Subjective constancy or perceptual constancy is the perception of an object or quality as constant under changing conditions.

In vision this means perceiving a color as "constant under changing conditions of illumination" (Erickson 1975, p.11-12) and "is the achievement of a very complicated 'calculation' by an unconsciously working apparatus within our central nervous system" (Lorenz 1961, p.171).

In music, subjective constancy is the identification of a musical instrument as constant under changing timbre or "conditions of changing pitch and loudness, in different environments and with different players." (Erickson ibid)

In speech perception this means that vowels or consonants are perceived as constant categories even if acoustically, they vary greatly due to phonetic environment (coarticulation), speech tempo, speaker's age and sex, speaker's dialect, etc.