Source : Free On-Line Dictionary of Computing

tweening
     
         An {interpolation} technique where an {animation}
        program generates extra frames between the key frames that the
        user has created.  This gives smoother animation without the
        user having to draw every frame.
     
        A scene is described by a mathematical model - a set of two-
        or three-dimensional objects whose positions in are given by
        sets of coordinates.  Tweening uses mathematical formulae to
        generate these coordinates at a sequence of discrete times.
        The simplest system would move each point at a constant rate
        in a straight line between its initial and final positions,
        though other kinds of path are possible.  The coordinates at
        each time step are used to generate (or "render") a
        two-dimensional image of the scene which forms one "frame" of
        the animation.
     
        Tweening is similar to {morphing} except that morphing is
        usually performed by interpolating between corresponding
        points marked by the user on two images, rather than between
        two configurations of a model.
     
        (1995-04-04)