a plan or intention.

1) In software development, a prototype is a rudimentary working model of a product or information system, usually built for demonstration purposes or as part of the development process.
Script/Synopsis/Treatment:

The character design is the process which comes after the characterisation and consists in defining the character through his/her physical appearance. We need to consider a character as a little fictional creature, human or not, that aims to please its creator and the public as well.
Concept art:

Concept art is a form of illustration used to convey an idea for use in films, video games, animation, comic books, or other media before it is put into the final product.

A storyboard is a graphic representation of how your video will unfold, shot by shot. It's made up of a number of squares with illustrations or pictures representing each shot, with notes about what's going on in the scene and what's being said in the script during that shot.