Wiktionary
n. A form of illustration whose main goal is to convey a visual representation of a concept.
Wikipedia
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. Concept art is also referred to as visual development and/or concept design. This term can also be applied to retail, set, fashion, architectural and industrial design.
Concept art is developed in several iterations. Artists try several designs to achieve the desired result for the work, or sometimes searching for an interesting result. Designs are filtered and refined in stages to narrow down the options. Concept art is not only used to develop the work, but also to show the project's progress to directors, clients and investors. Once the development of the work is complete, advertising materials often resemble concept art, although these are typically made specifically for this purposed, based on final work.
Usage examples of "concept art".
Some clients wanted me to do the concept art for the whole look of the game, which they'd then turn over to their in-house artists to base the games on.