This is a term that refers to an initial coat of paint that comes after the primer. It is most often associated with "candy" colors. The classic "candy" paint job is done by first applying a base coat of, for example, a metallic gold. This is then covered with semi-transparent color coats, in many instances layer after layer, until the desired effect is achieved.
The initial coat of paint applied to the surface. This is the most important layer as everything else sits on top of it so take special care to keep it smooth and even by using a thin coat (or coats if needed for opacity) of paint. Using a large brush will also help keep the base coat smooth as will stroking the paint in one direction.
A coat of paint applied after priming, that usually incorporates the overall color scheme of the model. For example, the base coat for a werewolf might be brown, and the base coat for a sea monster might be green.
A very light spray or troweled covering, applied with a "Wet Mix" and a high air pressure, used when matching all the concrete areas into one uniform color and for a different color "Grout Line". Would only be used when applying a "Top Coat". A "Base Coat" is not always required on all jobs.