TESL stands for Teaching English as a Second Language. It names the work of teaching English to people whose first language is something else — and, in its careful use, specifically to learners who are living in an English-speaking country, where English is the language outside the classroom door: a Vietnamese family in Australia, a Somali student in Canada. That is what 'second language' points at. This matters because of the sister term TEFL, Teaching English as a Foreign Language, which covers learners studying English in a place where it is not the everyday language — which is what most teaching in Taiwan, Japan or Korea actually is. In real life the two are often mixed up or used loosely, and many people now prefer the wider term TESOL, which covers both. You will mostly meet TESL in the names of university courses, certificates and job titles: a TESL certificate, a TESL programme. It is pronounced as a word, 'TESS-ul', though some people spell out the letters.