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.