Language is a system of communication using gestures, sounds, or written symbols that have significance for those who use the language and follow its rules. In speech, phonemes are the smallest units of sound in a language, and although they individually have no meaning, they acquire it when combined. For example, the phonemes k and r alone convey no meaning (other than that they are letters), but together they sound like car, which is a meaningful sound in the English language.

Semantics. The term semantics refers to the study of meaning in a language. The smallest unit of meaning in spoken language is called a morpheme, which in many instances is itself a word. The word overcoat is composed of two morphemes, over and coat. Language rules govern the combination of morphemes to create meaning; overcoat, for example, means something different than does coatover.

Sentences. Language rules also dictate syntax, how morphemes are put together to form sentences, groups of words that make meaningful statements. Pragmatics is the study of language as used in particular situations, which may affect its meaning. Consider the statement, “What a wonderful day!” and its various meanings if the sun is shining, if the rain is pouring down, and if the speaker has just received a traffic ticket.

Acquisition of language. B. F. Skinner believed that language acquisition, an important development in childhood, occurs because of reinforcement, that is, because childrens' parents or other caregivers reward them when their initially random sounds most resemble speech. Linguist Noam Chomsky contested Skinner's approach and proposed the well‐known, but controversial, theory that children have an innate neural mechanism called a language acquisition device (LAD) (not yet discovered), which allows them to master language.

Developmental psychologists have subsequently documented the general process of language acquisition, which is usually thought to progress through the stages shown in Table 1.