ESL is the popular term for English as a Second Language. It is used when referring to the teaching of English, in an English-speaking country, to people whose native language is not English.

ESOL is the acronym for English for Speakers of Other Languages or for teaching English to Speakers of Other Languages. Some people prefer this term to ESL, as it takes into consideration that some learners may already speak a second language and are now learning a third, fourth, or fifth language.

EFL stands for English as a Foreign Language and refers to the teaching and learning of English in countries where English is not the official native language.

ELL stands for English language learner. This phrase is frequently used to describe nonnative English speakers in K-12 education.

SLA stands for second language acquisition, and SLA research focuses on the processes and outcomes of learning a second language.

The National Reporting System (NRS) establishes descriptors for functioning levels of adult ESL learners. The levels include, from lowest to highest levels, Beginning ESL Literacy, Beginning ESL, Low Intermediate ESL, High Intermediate ESL, Low Advanced ESL, and High Advanced ESL. (See National Reporting System (NRS) ESL functioning level descriptors.)

More than 35 million adults in the United States are native speakers of a language other than English. (See Numbers.)

The foreign-born population in the United States comes from all over the world. The largest group of immigrants comes from Mexico and other Latin American countries. The next largest group comes from countries in Asia. (See Countries of Origin.)