Motion tweening is a process in animation that creates inbetween frames without having to manually draw new frames. A common form of tweening is puppet animation, which involves cutting a character into parts, rigging them together, and then moving them one at a time. As such, the tweening process is not fully automatic most of the time, instead, it's careful manipulation of an existing image (or group of images) to add the illusion of movement.
This tag is not mutually exclusive with frame by frame, the two often go hand-in-hand and both will be used in the same animation. However, you shouldn't tag both if one is just being used for minor elements. Examples of mixing tweening and frame-by-frame
See also