1. an early stage in life when an organism is especially open to specific learning, emotional, or socializing experiences that occur as part of normal development and will not recur at a later stage. For example, the first 3 days of life are thought to constitute a critical period for imprinting in ducks, and there may be a critical period for language acquisition in human infants. See also sensitive period.

2. in vision, the period of time after birth, varying from weeks (in cats) to months (in humans), in which full, binocular visual stimulation is necessary for the structural and functional maturation of the visual system. See also monocular rearing.