Real Dictionary

hammer
countable noun: A hammer is a tool that consists of a heavy piece of metal at the end of a handle. It is used, for example, to hit nails into a piece of wood or a wall, or to break things into pieces.
verb: If you hammer an object such as a nail, you hit it with a hammer.

hammer in American English
a tool for pounding, usually consisting of a metal head fastened across one end of a handle: one end of the head may be a pronged claw for pulling nails
to strike repeatedly with or as with a hammer
noun: a tool consisting of a solid head, usually of metal, set crosswise on a handle, used for beating metals, driving nails, etc

hammer in British English
noun: a hand tool consisting of a heavy usually steel head held transversely on the end of a handle, used for driving in nails, beating metal, etc