It equals 1000 pounds-force, and is used primarily by structural engineers to indicate forces where the value represented in pound-force is inefficient.
Although uncommon, it is occasionally also considered a unit of mass, equal to 1000 pounds (i.e. one half of a short ton).
The name comes from combining the words kilo and pound; it is occasionally called a kilopound.
Its symbol is kip, sometimes K (upper or lowercase), or less frequently, klb.
When it is necessary to clearly distinguish it as a unit of force rather than mass, it is sometimes called the kip-force (symbol kipf or klbf).