Real Dictionary

nail
countable noun: A nail is a thin piece of metal with one pointed end and one flat end. You hit the flat end with a hammer in order to push the nail into something such as a wall.
countable noun: Your nails are the thin hard parts that grow at the ends of your fingers and toes.
verb: If you nail something somewhere, you fix it there using one or more nails.
verb: To nail someone means to catch them and prove that they have been breaking the law.

nail in American English
a thin, horny covering that grows out over the upper tip of a finger or toe
to attach or fasten together or onto something else with or as with nails
noun: a slender, typically rod-shaped rigid piece of metal, usually in any of numerous standard lengths from a fraction of an inch to several inches and having one end pointed and the other enlarged and flattened, for hammering into or through wood, other build

nail in British English
noun: a fastening device usually made from round or oval wire, having a point at one end and a head at the other