The Free On-line Dictionary of Computing (30 December 2018):

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)