Definition of "flathead"

Any fish in the family Platycephalidae.  examples

(US, informal) Alternative form of flathead catfish  examples

(plural only "flatheads") A type of screw or bolt designed to fit in a countersink so that it sits flush with a surface.  examples

(plural only "flatheads", automotive) A type of engine that has the valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine.  examples