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.

If you nail something somewhere, you fix it there using one or more nails.

Your nails are the thin hard parts that grow at the ends of your fingers and toes.

To nail someone means to catch them and prove that they have been breaking the law.

If you say that someone is as hard as nails, you mean that they are extremely tough and aggressive, either physically or in their attitude towards other people or other situations.

If you say that someone has hit the nail on the head, you think they are exactly right about something.