The project presented in this book is a longitudinal multi-individual study of the development of Swedish as a second or, to be more precise, a third language. Research on third/next language acquisition is not new in linguistic studies. It has grown as a branch of second language studies due to the increasingly widespread view that multilingualism is at least as common as monolingualism (Aronin & Singleton, 2008; Cook, 1992; Grosjean, 1982) and in many cases studies referred to as ‘second language investigations’ actually explore the development and use of a third or fourth language. For example, we can with a high degree of probability assume that the Dutch learner of Finnish in the study of Spoelman and Verspoor (2010) described above knew at least one more language, due to the fact that English is a compulsory subject in Dutch schools and Finnish was thus at least his third language. The term second language most often occurs in monolingual communities in contexts where English is learned as a foreign language at school, in countries where there is an official language but in which co-official languages are also represented (e.g. as in Spain) or in the case of languages learned by uneducated immigrants originating from monolingual regions. In all other settings the learner in fact acquires a third, fourth or fifth language. The expression third language can mean an individual's third language, ordered chronologically, or a third and every subsequently acquired language, called L ≥ 3 (Fouser, 2001) or a “third or additional language” (de Angelis, 2007). In many studies, the following definition, devised by Hammarberg (2010, p. 97), is used:

However, even if such a distinction (between second and third language) is made, researchers often use the term second language even in those contexts where it is actually the development of a subsequent, i.e. third, fourth and so on, language that is being investigated. The term third language, on the other hand, occurs in settings where the focus is on the phenomenon of multilingualism and the interplay of the learner's languages.x