Cruise missiles can be launched from ships, subs and planes
Cruise missiles have been a key part of major American military campaigns since the 1991 Gulf War.
But the term itself is generic, referring to self-propelled guided weapons which fly like normal aircraft for most of their flight.
The simplest cruise missiles (developed by the Chinese) have a range of approximately 60 miles (100 km).
The US arsenal includes weapons that can be fired at targets from up to 1,500 miles (2,413 km) away yet land with a claimed accuracy of a few metres.
Each costs more than $1m, although planned new Tomahawk missiles will be less than $600,000 apiece.
Sea-launched Tomahawks receive an initial thrust from a detachable rocket booster before the onboard turbofan engine takes over.
A bigger, air-launched version - which needs no booster - can be carried by American B-52 bombers.
Once launched, a cruise missile unfolds its wings and tail fins and switches on navigational and communication systems.
US cruise missiles are designed to hug the terrain to evade radar. They are difficult to stop, especially if launched in batches, due to their relatively small size - although they travel at only about 550mph.
Newer versions incorporate GPS satellite guidance.