Modulation

A work of tonal music modulates when it changes key. Typically, this is achieved by a the use of a cadence in the new key, in particular a movement from the dominant seventh of the new key to the tonic. A cadence is essentially a musical grammatical mark, like a period, denoting the end of an idea. Additionally, a cadence has a strong key defining effect. So if a composer can smoothly work in a cadence from the new key, there is a sense of continuity between the new and original key.