ESL means English as a Second Language: the teaching of English to people who are living in an English-speaking country but whose first language is not English. A child who moves to the United States or Britain and then learns English at school is in that kind of class. Though actually, the letters get used loosely. US schools say ESL a lot; Britain often says ESOL or EAL instead, and some places now say EL or ELL, “English learner.” This is the bit Taiwan students mix up: if you learn English in Taiwan, you are usually doing EFL — English as a Foreign Language — because English is not the language of the street here. ESL is for when English is all around you outside class. The two overlap in real life, and some teachers just say “English teaching” to avoid the fight. ESL is a normal school and university word, not slang.