What should one understand by the term "bilingualism"?
Bilingualism is the ability of a person to speak two languages equally well, with the second one learned at the same time as or after the first or native language is learned. Often a person becomes bilingual when raised in a home in which a language is spoken that is different from the language used in schools. For instance, in India, many children grow up speaking Hindi at home but then learn and use English exclusively at school because English is the language used for education.
Bilingualism can also occur when a child's parents each speak a different language. For example, if one parent speaks Hindi and the other parent speaks French at home, the child will grow up hearing and responding to both and will thus become bilingual. There are many ways in which bilingualism can develop. Perhaps a grandparent speaks a language other than the one the parents speak; perhaps a child's parents are immigrants and their native language is spoken exclusively by the child until school education conducted in a second language begins.
Another way to become bilingual is for a person to learn and acquire a second language as an adult. This happens frequently in American high schools where second languages are taught. Another possibility is that an adult may move to a country where a different language is spoken, then learn the new language for work or socialization. In contrast, knowing more than two languages is called multilingualism, while knowing only one language is called monolingualism.