Babbling  -  Pre-speech vocalizations made by infants around nine months old.

Language Acquisition Device  -  A innate "mental module" postulated by Noam Chomsky to explain the ability of children to learn despite the "poverty of the stimulus."

Naming Burst  -  A dramatic increase in the rate of word learning that takes place around two to three years of age.

Overextension  -  When children use a word to refer to a class of objects that is broader than in adult usage. For instance, using "dog" to refer to all four- legged creatures.

Overgeneralization  -  When children apply a grammatical rule across all members of a grammatical class (e.g. verbs) without making the appropriate exceptions. For instance, using the -ed suffix to indicate past tense for verbs like "go" and "think."

Poverty of the Stimulus  -  Term used by Noam Chomsky to describe the impossibility of learning language from verbal input without some kind of innate constraints on the kind of language that can be learned.

Underextension  -  When children use a word to refer to a class of objects that is narrower than in adult usage. For instance, using "dog" to refer only to German shepherds.

Williams Syndrome  -  A genetic disorder that results in elfin-like facial characteristics, often severe mental retardation, and a surprising sociability and facility with language.