Final answer:
During the sensorimotor stage of cognitive development, infants learn object permanence, which is the understanding that objects continue to exist even when out of sight.
Step-by-step explanation:
During the sensorimotor stage of cognitive development, infants learn about the world through their senses and motor behaviors. A key concept developed during this stage is object permanence, which is the understanding that objects continue to exist even when they can no longer be seen or heard. This stage, as proposed by Jean Piaget, lasts from birth to about 2 years old, and it is a period of rapid cognitive growth. By around 8 months old, most children begin to grasp that objects are still present even when they are out of sight, indicating they have developed object permanence.