But "transposing instrument" is the musical classification assigned to those instruments for which music must be transposed for it to be heard at concert pitch.

Much though I dislike using Wikipedia for, well, anything really, the definition at the head of the 'transposing instruments' article puts it pretty well:

A transposing instrument is a musical instrument for which written notes are read at a pitch different from the corresponding concert pitch, which a non-transposing instrument, such as a piano, would play. Playing a written C on a transposing instrument will produce (sound) a note other than concert C. The concert pitch of that written C determines the key from which an instrument transposes.

http://en.wikipedia.org/wiki/Transposing_instrument