Second-language acquisition (SLA), sometimes called second-language learning — otherwise referred to as L2 (language 2) acquisition, is the process by which people learn a second language. Second-language acquisition is also the scientific discipline devoted to studying that process. The field of second-language acquisition is regarded by some but not everybody as a sub-discipline of applied linguistics but also receives research attention from a variety of other disciplines, such as psychology and education. A central theme in SLA research is that of interlanguage: the idea that the language that learners use is not simply the result of differences between the languages that they already know and the language that they are learning, but a complete language system in its own right, with its own systematic rules. This interlanguage gradually develops as learners are exposed to the targeted language. The order in which learners acquire features of their new language stays remarkably constant, even for learners with different native languages and regardless of whether they have had language instruction. However, languages that learners already know can have a significant influence on the process of learning a new one. This influence is known as language transfer. The primary factor driving SLA appears to be the language input that learners receive. Learners become more advanced the longer they are immersed in the language they are learning and the more time they spend voluntarily reading. The input hypothesis developed by linguist Stephen Krashen theorizes that comprehensible input alone is necessary for second language acquisition. Krashen makes a distinction between language acquisition and language learning (the acquisition–learning distinction), claiming that acquisition is a subconscious process, whereas learning is a conscious one. According to this hypothesis, the acquisition process in L2 (Language 2) is the same as L1 (Language 1) acquisition. Learning, on the other hand, refers to conscious learning and analysis of the language being learned. Krashen argues that consciously learned language rules play a limited role in language use, serving as a monitor that could check second language output for form — assuming the learner has time, sufficient knowledge, and inclination (the monitor hypothesis). Subsequent work, by other researchers, on the interaction hypothesis and the comprehensible output hypothesis, has suggested that opportunities for output and interaction may also be necessary for learners to reach more advanced levels. Research on how exactly learners acquire a new language spans several different areas. Focus is directed toward providing