Object Permanence: The understanding that an object continues to exist even when it is not visible. For example, knowing that the toy is still present even if it's hidden behind a pillow.
Sensorimotor Stage: The stage of development (from birth to about 2 years old) in Piaget's theory where infants learn about the world through their senses and actions.
Preoperational Stage: The second stage of Piaget's theory (from about 2 to 7 years old) characterized by egocentrism and the use of symbols such as language and pretend play.