In developmental psychology and developmental biology, a critical period is a maturational stage in the lifespan of an organism during which the nervous system is especially sensitive to certain environmental stimuli. If, for some reason, the organism does not receive the appropriate stimulus during this "critical period" to learn a given skill or trait, it may be difficult, ultimately less successful, or even impossible, to develop some functions later in life. Functions that are indispensable to an organism's survival, such as vision, are particularly likely to develop during critical periods. "Critical period" also relate to ability to acquire first language. Researchers found that people who passed "critical period" would not acquire first language fluently.
Some researchers differentiate between 'critical' and 'sensitive' periods—defining 'sensitive' periods as more extended periods, after which learning is still possible. Other researchers consider these the same phenomenon.
For example, the critical period for the development of a human child's binocular vision is thought to be between three and eight months, with sensitivity to damage extending up to at least three years of age.