A spear-thrower or atlatl (/ˈætlætəl/ or /ˈɑːtlɑːtəl/; Nahuatl languages: ahtlatl; Nahuatl pronunciation: [ˈaʔt͡ɬat͡ɬ]) is a tool that uses leverage to achieve greater velocity in dart-throwing, and includes a bearing surface which allows the user to store energy during the throw.
It may consist of a shaft with a cup or a spur at the end that supports and propels the butt of the dart. The spear-thrower is held in one hand, gripped near the end farthest from the cup. The dart is thrown by the action of the upper arm and wrist. The throwing arm together with the atlatl acts as a lever. The spear-thrower is a low-mass, fast-moving extension of the throwing arm, increasing the length of the lever. This extra length allows the thrower to impart force to the dart over a longer distance, thus imparting more energy and ultimately higher speeds.
Common modern ball throwers (molded plastic shafts used for throwing tennis balls for dogs to fetch) use the same principle.
A spear-thrower is a long-range weapon and can readily impart to a projectile speeds of over 150 km/h (93 mph).
Spear-throwers appear very early in human history in several parts of the world, and have survived in use in traditional societies until the present day, as well as being revived in recent years for sporting purposes. In the United States the Nahuatl word atlatl is often used for revived uses of spear-throwers, and in Australia the Aboriginal word woomera.
The ancient Greeks and Romans used a leather thong or loop, known as an ankule or amentum, as a spear-throwing device.