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

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.

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.

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.

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

A perspective that compares human thinking processes, by analogy, to computer analysis of data, including sensory input connections, stored memories, and output.

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

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

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

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

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

The average number of words and meaningful sounds (such as -ing and huh?) 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.

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

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.

Perception that is primed to focus on movement and change.

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.

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

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

A universal principle of infant perception, specifically an innate attraction to other humans, evident in visual, auditory, and other preferences.

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