French Flag is a light blocking device that allows you to limit or block light spilling directly into the camera lens, in order to reduce flares.
The french flag is either mounted directly to the top of the camera's matte box, or mounted via clip and arm for adjustable positioning.
A Flag is an opaque object that is used to block light or to create a shadow from a light emmiting source.
A flag is also referred to as a Solid. Most often a flag is made of black, fire-retardant duvatine cloth wrapped around a wire rod frame with a mounting pin.