Every normal child acquires a language, his first language (or ‘native tongue’), in the first few years of life. There are exceptions, on either physiological (e.g. deafness) or social grounds (e.g. ‘wolf children’); but usually a child can communicate freely by the time he goes to school. Beyond puberty, our command of language shows little progress, though in some areas – the vocabulary, for instance – learning continues throughout our life span. First language acquisition is thus primary in at least two ways: in terms of sequence (‘first’) and in terms of (mostly life-long) importance.

Most people learn more than one language, however. There are various ways in which this may happen, and the transitions between them are gradual. A child may be exposed to two (or even more) languages right from the beginning, for example if his parents use different languages. In this case, we may still speak of ‘first language acquisition’ – except that not one but two languages are ‘first’. In other words, a language is ‘first’ – and so is its acquisition – if no other language was acquired before; otherwise, it is second. The distinction is neat if acquisition of the second language begins when acquisition of the first is over, as is typically the case after puberty. But since the acquisition process extends over a long period of time, there are all sorts of intermediate cases.