Transposition is the raising or lowering of all the notes of an entire piece or of a section of it by a constant interval, either an octave, thus without changing the key, or some other interval, and thus changing the key. Transpositions can be made for scores, or they can be carried out on the fly by musicians as they perform.
A major reason for transposing a piece is in order to accommodate the vocal range of a singer or the range of a specific type of instrument. Another reason is that some conductors or performers may prefer the way a piece sounds in a different key. Also, transposition of a section of a piece is sometimes used as a means of creating variations within the piece. In addition, composers will sometimes use successive transpositions in a piece to build psychological intensity.