Second-language acquisition or second-language learning is the process by which people learn a second language. Second-language acquisition (often abbreviated to SLA) also refers to the scientific discipline devoted to studying that process. Second language refers to any language learned in addition to a person's first language; although the concept is named second-language acquisition, it can also incorporate the learning of third, fourth, or subsequent languages.[1] Second-language acquisition refers to what learners do; it does not refer to practices in language teaching. The academic discipline of second-language acquisition is a subdiscipline of applied linguistics. It is broad-based and relatively new. As well as the various branches of linguistics, second-language acquisition is also closely related to psychology, cognitive psychology, and education. To separate the academic discipline from the learning process itself, the terms second-language acquisition research, second-language studies, and second-language acquisition studies are also used. SLA research began as an interdisciplinary field, and because of this it is difficult to identify a precise starting date. However, it does appear to have developed a great deal since the mid-1960s.[2] The term acquisition was originally used to emphasize the subconscious nature of the learning process,[3] but in recent years learning and acquisition have become largely synonymous. Second-language acquisition can incorporate heritage language learning,[4] but it does not usually incorporate bilingualism. Most SLA researchers see bilingualism as being the end result of learning a language, not the process itself, and see the term as referring to native-like fluency. Writers in fields such as education and psychology, however, often use bilingualism loosely to refer to all forms of multilingualism.[5] Second-language acquisition is also not to be contrasted with the acquisition of a foreign language; rather, the learning of second languages and the learning of foreign languages involve the same fundamental processes in different situations.[6]. There has been much debate about exactly how language is learned, and many issues are still unresolved. There are many theories of second-language acquisition, but none are accepted as a complete explanation by all SLA researchers. Due to the interdisciplinary nature of the field of second-language acquisition, this is not expected to happen in the foreseeable future. As SLA began as an interdisciplinary field, it is hard to pin down a precise starting date.[2] However, there are two publications in particular that are seen as instrumental to the development of the modern study of SLA: Pitt Corder's 1967 essay The Significance of Learners' Errors, and Larry Selinker's 1972 article Interlanguage. Corder's essay rejected a behaviorist account of SLA and suggested that learners made use of intrinsic internal linguistic processes; Selinker's article argued that second language learners possess their own individual linguistic systems that are independent from both the first and second languages.[7] In the 1970s the general trend in SLA was for research exploring the ideas of Corder and Selinker, and refuting behaviorist theories of language acquisition. Examples include research into error analysis, studies in transitional stages of second-language ability, and the "morpheme studies" investigating the order in which learners acquired linguistic features. The 70s were dominated by naturalistic studies of people learning English as a second language.[7] By the 1980s, the theories of Stephen Krashen had become the prominent paradigm in SLA. In his theories, often collectively known as the Input Hypothesis, Krashen suggested that language acquisition is driven solely by comprehensible input, language input that learners can understand. Krashen's model was influential in the field of SLA and also had a large influence on language teaching, but it left some important processes in SLA unexplained. Research in the 1980s was characterized by the attempt to fill in these gaps. Some approaches included Lydia White's descriptions of learner competence, and Manfred Pienemann's use of speech processing models and lexical functional grammar to explain learner output. This period also saw the beginning of approaches based in other disciplines, such as the psychological approach of connectionism.[7] The 1990s saw a host of new theories introduced to the field, such as Michael Long's interaction hypothesis, Merrill Swain's output hypothesis, and Richard Schmidt's noticing hypothesis. However, the two main areas of research interest were linguistic theories of SLA based upon Noam Chomsky's universal grammar, and psychological approaches such as skill acquisition theory and connectionism. The latter category also saw the new theories of processability and input processing in this time period. The 1990s also saw the introduction of sociocultural theory, an approach to explain second-language acquisition in terms of the social environment of the learner.[7] In the 2000s research was focused on much the same areas as in the 1990s, with research split into two main camps of linguistic and psychological approaches. VanPatten and Benati do not see this state of affairs as changing in the near future, pointing to the support both areas of research have in the wider fields of linguistics and psychology, respectively.