A motion tween is a feature available in Adobe Flash (formerly Macromedia Flash) that allows you to easily animate the motion of an object. Instead of defining the location of the object in every frame, you can create a motion tween, which will automatically move the object from the beginning location to ending location.
To create a motion tween, simply select a layer in the timeline and drag an object onto the stage. Then select the number of frames in the timeline you would to use for the duration of the animation. To create the motion tween, you can either right-click in the timeline and select "Create Motion Tween," or simply choose from the menu bar. NOTE: In order for Flash to create the tween, you may need to convert the object to a symbol.
Once the tween has been created, you can click on any frame within the motion tween and move or rotate the object. For example, you can click on the last frame in the motion tween and move the object to a different part of the stage. When you run the animation, Flash will automatically calculate the location of the symbol for each frame and smoothly move the symbol from the start location to the end location. You can modify the acceleration of the object using the "Ease" property in the Properties palette.
Motion tweening has become the standard way of animating symbols in Flash animations. While the name "motion tween" is specific to Adobe Flash, the phrase is sometimes used to refer to automated movements in other animation software as well.