In-between

Once the key frames of an animation are created, junior level artists (sometimes at a different studio in another country) go in and add frames between the key frames to make the motion smoother - hence the name in-betweens (or tweens for short). These are often looser and less on model since they just help move from one important key frame to another and also due to the lower skill level of those junior animators.

In-betweening is widely regarded as drudge work in the animation industry. This is where inexperienced animators begin their career but it is repetitive, uncreative work, and the inbetweeners often have extremely high production quotas to be met. In-betweening is often subcontracted to the lowest bidder, which can result in shoddy work.