Language Learning

Second-language acquisition, second-language learning, or L2 acquisition, is the process by which people learn a second language. Second-language acquisition (often abbreviated to SLA) also refers to the scientific discipline devoted to studying that process.

Second language refers to any language learned in addition to a person's first language; although the concept is named second-language acquisition, it can also incorporate the learning of third, fourth, or subsequent languages.
Second-language acquisition refers to what learners do; it does not refer to practices in language learning.

http://en.wikipedia.org/wiki/Second-language_acquisition