Sequential bilingualism occurs when a person becomes bilingual by first learning one language

Language

Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

and then another.
There is variation in the period in which learning must take place for bilingualism to be considered simultaneous. Generally, the term sequential bilingualism applies only if the child is approximately three years old before being introduced to the second language.