Has multiple definitions:

• education where two distinct languages are used for general teaching;
• education designed to help children become bilingual (sometimes called “two-way bilingual education”; e.g., Spanish speakers and English speakers in a classroom are all taught to speak both languages;
• education in a child’s native language for (a) the first year or (b) however long it takes; followed by mainstreaming in English-only classes (in the US);
• education in a child’s native language for as long as his parents wish (with minimal instruction in another language).

In the latter cases “native-language instruction” may be a clearer definition.