Second-language acquisition is the learning of a language other than one's:

1. A second language is any language learned in addition to the first. 2. Second-language acquisition is the process of learning a language other than one's native language (L1). 3. So the language set aside is the native language. **Why not the others:** A regional dialect, an official language and a classical language are not the baseline; the baseline is the native language. _Source: Wikipedia, "Second-language acquisition", section "introduction" (revision 1380191148)_