A study of second language acquisition in two Japanese children, and corresponding examination of research literature, led to this effort to clarify terminology related to cross-linguistic influence in language contact situations. Terms include: bilingualism, code-switching, language mixing, language transfer, and borrowing. Two forms of transfer are distinguished: borrowing (the influence of a second language on a previously acquired language) and substratum (the influence of a native language or some other previously learned language on acquisition of another language). Borrowing refers to lexical borrowing, usually confined to single-item terms but sometimes extended to phrase- and sentence-level constructions. Code-switching is the alternation of languages within a single discourse, sentence, or constituent, and is rule-governed and characterized by social functions. Several theorists distinguish between kinds of code-switching: situational vs. conversational; emblematic vs. intimate; and matrix language vs. embedded language. Three distinct models of bilingualism are identified: (1) one positing two monolingual grammars and a separate grammar of code-switching; (2) two monolingual grammars combined in code-switching; and (3) two monolingual grammars used by a processor with a separated code-switching mechanism. Contains 39 references. (MSE)