Settei Sets/Model Sheets (設定)
Settei/Models (設定) are drawn by a character designer and define what a character should look like. Everyone involved in production (primary animators, "in-betweeners", directors, etc.) will reference settei/models to ensure that the characters look uniform. When the animation is consistent, this is often referred to as being "on model." Conversely, when someone says that a portion of animation is "off model", it means that it does not match what the character designer intended.
Collector's Note: You will most often find these as photocopies. The hand drawn sets will come up occasionally, but they are rare and often expensive.