What do you call a flat cone?

A flat cone is typically referred to as a "conical frustum" when it has a truncated apex, or simply a "cone" if it is understood to be very shallow. In a more general sense, it can also be described as a "pyramid with a circular base" if the cone is extremely flattened. Additionally, in some contexts, it may be colloquially referred to as a "disk" or "plate."