Tweening, short for in-betweening, is the process of generating intermediate frames between two images to give the appearance that the first image evolves smoothly into the second image. Inbetweens are the drawings between the keyframes which help to create the illusion of motion. Tweening is a key process in all types of animation, including computer animation. Sophisticated animation software enables one to identify specific objects in an image and define how they should move and change during the tweening process. Software may be used to manually render or adjust transitional frames by hand or may be used to automatically render transitional frames using interpolation of graphic parameters.
In the inbetweening workflow of traditional hand-drawn animation, the senior or key artist would draw the keyframes which define the movement, then, after testing and approval of the rough animation, would hand over the scene to his or her assistant. The assistant does the clean-up and the necessary inbetweens, or, in large studios, only some breakdowns which define the movement in more detail, then handing down the scene to his assistant, the inbetweener who does the rest.
Contrary to popular belief, no animator really draws inbetweens for all 24 frames required for one second of film. Only very fast movements require animation "on ones", as it is called. Most movements can be done with 12 drawings per second, which is called animating "on twos". Too few inbetweens distort the illusion of movement, like in cheap TV animation series where there can be as few as 4 drawings for a second of film. Computer generated animation is always animated on ones. The decision about the number of inbetweens is also an artistic one, as certain styles of animation require a not-so-smooth fashion of movement.
Traditional inbetweening involves the use of light tables to draw a set of pencil-on-paper pictures, but in more recent times computers may be used to speed up the inbetweening process. The use of computers for inbetweening was pioneered by Nestor Burtnyk and Marceli Wein at the National Research Council of Canada. They received a Technical Achievement Academy Award in 1997, for "pioneering work in the development of software techniques for computer assisted key framing for character animation".
In the context of Adobe Flash, the process is simply called "tweening," and the resultant animation is called a "tween."