In animation, Inbetweening, tweening or Interpolation is the process of generating intermediate frames between two images, called key frame, to give the appearance that the first image evolves smoothly into the second image.
A set of key points for the starting key frame and for the ending key frame is defined. (for instance, the path of an object) The computed interpolation process is then used to insert many new values in between these key points to give a smoother result. See Data Viz - (Timing|Easing) Function
wiki/keyframing, the properties of an object can be specified at certain points in time by setting a series of keyframes so that the properties of the object can be automatically altered (or tweened) in the frames between keyframes.