A second language (L2) is a language spoken in addition to one's first language (L1). A second language may be a neighbouring language, another language of the speaker's home country, or a foreign language.

A speaker's dominant language, which is the language a speaker uses most or is most comfortable with, is not necessarily the speaker's first language. For example, the Canadian census defines first language for its purposes as "What is the language that this person first learned at home in childhood and still understands?", recognizing that for some, the earliest language may be lost, a process known as language attrition. This can happen when young children start school or move to a new language environment.

The distinction between acquiring and learning was made by Stephen Krashen as part of his monitor theory. According to Krashen, the acquisition of a language is a natural process; whereas learning a language is a conscious one. In the former, the student needs to partake in natural communicative situations. In the latter, error correction is present, as is the study of grammatical rules isolated from natural language. Not all educators in second language agree to this distinction; however, the study of how a second language is learned/acquired is referred to as second-language acquisition (SLA).

Research in SLA "...focuses on the developing knowledge and use of a language by children and adults who already know at least one other language... [and] a knowledge of second-language acquisition may help educational policy makers set more realistic goals for programmes for both foreign language courses and the learning of the majority language by minority language children and adults."

SLA has been influenced by both linguistic and psychological theories. One of the dominant linguistic theories hypothesizes that a device or module of sorts in the brain contains innate knowledge. Many psychological theories, on the other hand, hypothesize that cognitive mechanisms, responsible for much of human learning, process language.

Other dominant theories and points of research include 2nd language acquisition studies (which examine if L1 findings can be transferred to L2 learning), verbal behaviour (the view that constructed linguistic stimuli can create a desired speech response), morpheme studies, behaviourism, error analysis, stages and order of acquisition, structuralism (approach that looks at how the basic units of language relate to each other according to their common characteristics), 1st language acquisition studies, contrastive analysis (approach where languages are examined in terms of differences and similarities) and inter-language (which describes the L2 learner's language as a rule-governed, dynamic system).

These theories have all influenced second-language teaching and pedagogy. There are many different methods of second-language teaching, many of which stem directly from a particular theory. Common methods are the grammar-translation method, the direct method, the audio-lingual method (clearly influenced by audio-lingual research and the behaviourist approach), the Silent Way, suggestopedia, community language learning, the total physical response method, and the communicative approach (highly influenced by Krashen's theories). Some of these approaches are more popular than others, and are viewed to be more effective. Most language teachers do not use one singular style, but will use a mix in their teaching. This provides a more balanced approach to teaching and helps students of a variety of learning styles succeed.

The defining difference between a first language (L1) and a second language (L2) is the age the person learned the language. For example, linguist Eric Lenneberg used second language to mean a language consciously acquired or used by its speaker after puberty. In most cases, people never achieve the same level of fluency and comprehension in their second languages as in their first language. These views are closely associated with the critical period hypothesis.