English as a second or foreign language is the use of English by speakers with different native languages. Language education for people learning English may be known as English as a second language (ESL), English as a foreign language (EFL), English as an additional language (EAL), English as a New Language (ENL), or English for speakers of other languages (ESOL). The aspect in which ESL is taught is referred to as teaching English as a foreign language (TEFL), teaching English as a second language (TESL) or teaching English to speakers of other languages (TESOL). Technically, TEFL refers to English language teaching in a country where English is not the official language, TESL refers to teaching English to non-native English speakers in a native English-speaking country and TESOL covers both. In practice, however, each of these terms tends to be used more generically across the full field. TEFL is more widely used in the UK and TESL or TESOL in the US.

The term "ESL" has been seen by some to indicate that English would be of subordinate importance; or example, where English is used as a lingua franca in a multilingual country. The term can be a misnomer for some students who have learned several languages before learning English. The terms "English language learners" (ELL), and, more recently, "English learners" (EL), have been used instead, and the students' native languages and cultures are considered important.

Methods of learning English are highly variable, depending on the student's level of English proficiency and the manner and setting in which they are taught, which can range from required classes in school to self-directed study at home, or a blended combination of both. In some programs, educational materials (including spoken lectures and written assignments) are provided in a mixture of English, and the student's native language. In other programs, educational materials are always in English, but the vocabulary, grammar, and context clues may be modified to be more easily understood by students with varying levels of comprehension. Adapting comprehension, insight-oriented repetitions, and recasts are some of the methods used in training. However, without proper cultural immersion (social learning grounds) the associated language habits and reference points (internal mechanisms) of the host country are not completely transferred through these programs. As a further complication, the syntax of the language is based on Latin grammar hence it suffers inconsistencies. The major engines that influence the language are the United States and the United Kingdom and they both have assimilated the language differently so they differ in expressions and usage. This is found to a great extent primarily in pronunciation and vocabulary. Variants of the English language also exist in both of these countries (e.g. African American Vernacular English).

The English language has a great reach and influence, and English is taught all over the world. In countries where English is not usually a native language, there are two distinct models for teaching English: educational programs for students who want to move to English-speaking countries, and other programs for students who do not intend to move but who want to understand English content for the purposes of education, entertainment, employment or conducting international business. The differences between these two models of English language education have grown larger over time, and teachers focusing on each model have used different terminology, received different training, and formed separate professional associations. English is also taught as a second language for recent immigrants to English-speaking countries, which faces separate challenges because the students in one class may speak many different native languages.