A transition is a passage of music composed to link one section of music to another. Transitions often function as a moment of transformation and may, or may not in themselves, introduce new, musical material.
Often in music, the transition is the middle section or formal function, while the main theme is the beginning, and the subordinate theme is the ending. It may traditionally be a part of the sonata form's exposition in which the composer modulates from the key of the first subject to the key of the second, though many Classical era works move straight from first to second subject groups without any transition.
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