English as a Second Language (ESL) refers to English taught in an English-speaking country. For example, if an Arabic-speaking student from Kuwait attends a university in Montana to improve her English while earning a degree in biomedical engineering or art or business, she will be learning in an ESL, not an EFL, context. When the student leaves the classroom, she will be functioning in an environment in which the primary language spoken is not the L1, with input in the target language, as Brown puts it, “readily available” (134). Teaching English as a Second Language is abbreviated as TESL and Teaching English to Speakers of Other Languages is abbreviated TESOL.
Brown, H. Douglas. Teaching By Principles. Pearson, 2007.