This chapter focuses on literature from a different field, second language acquisition (SLA), it is particularly important to attempt to sort out some of the terminological differences. It provides the conventional use of the words and terms in the SLA literature. SLA is not an idiosyncratic phenomenon, but develops in a systematic and often regular way that cuts across individual differences. The concept of bilingualism is interpreted differently in the field of SLA and fields such as psychology and education. It seems that most scholars use bilingualism to incorporate multilingualism in a way similar to which the term SLA includes third, fourth, and fifth language acquisition. The second/foreign language literature and the bilingualism literature often deal with the same or similar phenomena, but with different terminology. For second or foreign language research, the important issue is the exposure and use of the language in childhood.