A perceptual experience that helps a person recollect an idea, a thing, or an experience.

The process of becoming accustomed to an object or event through repeated exposure to it, and thus becoming less interested in it.

The stage-five toddler (age 12 to 18 months) who experiments without anticipating the results, using trial and error in active and creative exploration.

An infant’s repetition of certain syllables, such as ba-ba-ba, that begins when babies are between 6 and 9 months old.

An experimental apparatus that gives the illusion of a sudden drop-off between one horizontal surface and another.

The second of three types of feedback loops in sensorimotor intelligence, this one involving people and objects. Infants respond to other people, to toys, and to any other object they can touch or move.

The first of three types of feedback loops in sensorimotor intelligence, this one involving the infant’s own body. The infant senses motion, sucking, noise, and other stimuli and tries to understand them.

Piaget’s term for the way infants think—by using their senses and motor skills—during the first period of cognitive development.

The average number of words in a typical sentence (called utterance, because children may not talk in complete sentences). MLU is often used to indicate how advanced a child’s language development is.

Cells in an observer’s brain that respond to an action performed by someone else in the same way they would if the observer were actually performing that action.

Functional magnetic resonance imaging, a measuring technique in which the brain’s electrical excitement indicates activation anywhere in the brain; fMRI helps researchers locate neurological responses to stimuli.

A sequence in which an infant first perceives something done by someone else and then performs the same action hours or even days later.

Chomsky’s term for a hypothesized mental structure that enables humans to learn language, including the basic aspects of grammar, vocabulary, and intonation.

All the methods—word order, verb forms, and so on—that languages use to communicate meaning, apart from the words themselves.

A single word that is used to express a complete, meaningful thought.

Unconscious or automatic memory that is usually stored via habits, emotional responses, routine procedures, and various sensations.

The realization that objects (including people) still exist when they can no longer be seen, touched, or heard.

A sudden increase in an infant’s vocabulary, especially in the number of nouns, that begins at about 18 months of age.

The high-pitched, simplified, and repetitive way adults speak to infants and children. (Also called baby talk or motherese.)

The third of three types of feedback loops in sensorimotor intelligence, this one involving active exploration and experimentation. Infants explore a range of new activities, varying their responses as a way of learning about the world.

An opportunity for perception and interaction that is offered by a person, place, or object in the environment.

Memory that is easy to retrieve on demand (as in a specific test). Most explicit memory involves consciously learned words, data, and concepts.