A key visual is a piece of art released by an anime studio as a concept and showcase of how an upcoming anime project will look like, often a few months before its airing.
These are often in the form of posters with the copyright name prominently on it, focusing on the appearance of the main characters. Along with them are often released reference sheets for the same characters.
Note that this is different from concept art, which is instead for something that is/was still in production at the time of its creation.
Not to be confused with Key/VisualArt's.