The term "flathead" can refer to several different meanings depending on context.
1) In a general sense, it could be used as an adjective to describe something or someone with a flat head - such as a person with no hair or a character in a cartoon that has a square-shaped head.
2) It can also refer to a type of screw that has a recessed thread on one side, creating a flattened appearance.
3) In the context of fishing, "flathead" is used as a common name for several species of freshwater fish with flat heads and wide mouths - such as the channel catfish or the bullhead.