A technique used in music composition to make more melody of something one has already written. The previously composed phrase is rewritten backwards in a sense - Starting from the last note, the phrase is simply flipped. Note duration, articulation, and expression are conserved as the phrase is rewritten.
        _____            ____
       /     \          /    \ -this is a slur
-------------------|--------------------
                   |               
--------------|----|-----|--------------
  0           |    |     |        0
-|-----X------|----|-----|---X---|------
 |    |      X     |    X   |    |
-|----|------------|--------|----|------
      |            |        |
-------------------|--------------------

 f     p                          f

0s are half notes, Xs are quarter notes.

Usually, this technique is applied to a much more complicated phrase. Can sound quite nifty when combined with a sequence or inversion.