heritage language learning, but it does not usually incorporate bilingualism. Most SLA researchers see bilingualism as being the 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. SLA 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. Research background The academic discipline of second-language acquisition is a sub-discipline 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 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; because of this, it is difficult to identify a precise starting date. However, two papers in particular are seen as instrumental to the development of the modern study of SLA: Pit Corder's 1967 essay The Significance of Learners' Errors and Larry Selinker's 1972 article Interlanguage. The field saw a great deal of development in the following decades. Since the 1980s, SLA has been studied from a variety of disciplinary perspectives, and theoretical perspectives. In the early 2000s, some research suggested an equivalence between the acquisition of human languages and that of computer languages (e.g. Java) by children in the 5 to 11 year age window, though this has not been widely accepted amongst educators. Significant approaches in the field today are systemic functional linguistics, sociocultural theory, cognitive linguistics, Noam Chomsky's universal grammar, skill acquisition theory and connectionism. 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 SLA, this is not expected to happen in the foreseeable future. Although attempts have been made to provide a more unified account that tries to bridge first language acquisition and second language learning research. Stages Krashen stages Stephen Krashen divides the process of second-language acquisition into five stages: preproduction, early production, speech emergence, intermediate fluency, and advanced fluency. The