An appointment for a federal office made by the President when the Senate is not meeting. Recess appointments can serve until the end of the next session of the Senate. The President may nominate them again, but if they are not confirmed they must give up the post.
In the 19th century, this process enabled Presidents to keep the government functioning during the many months that Congress stood adjourned. In modern times, Congressional recesses are much shorter and Presidents have used recess appointments mostly for controversial nominees whose confirmations have been stalled.
By Donald Ritchie, Our Constitution (Oxford University Press)