TESL or teaching English as a second language refers to teaching English to students whose first language is not English and is usually offered in a region where English is the dominant language and natural English language immersion situations are apt to be plentiful.
TEFL or teaching English as a foreign language refers to teaching English to students whose first language is not English and usually in a region where English is not the dominant language and natural English language immersion situations are apt to be few.
These two teaching situations are different; so the teaching profession has different names for them.
Sievert, Jessica. "Evaluation of Structured English Immersion and Bilingual Education on Reading Skills of Limited English Proficient Students in California and Texas". Applied Research Project. Texas State University. 2007. Retrieved on 2008-07-04.