A leg is a limb on a living thing's body that supports the rest of the animal above the ground between the ankle and the hip and is used for locomotion. The end of the leg farthest from the animal's body is often either modified or attached to another structure that is modified to disperse the animal's weight on the ground (see foot). In bipedal vertebrate animals, the two lower limbs are usually referred to as the 'legs' and the two upper limbs as the 'arms' or 'wings' as the case may be.

In the anatomy of vertebrates, including human beings (see human leg), leg is also used to refer to the entire limb, but its precise definition refers only to the segment between the knee and the ankle. In vertebrate and human anatomy this segment is also called shank.

Legs typically come in even-numbered quantities. Many taxonomic groups are characterized by the number of legs their members possess.

• Uniped: 1
• Biped: 2
• Tripedal: 3
• Quadruped: 4
• Quinped: 5
• Arthropoda: 4, 6, 8, 12, or 14
• Some arthropods have more than a dozen legs; a few species possess over 100. Despite what their names might suggest,
• Centipedes typically have fewer than one hundred legs
• Millipedes have fewer than one thousand legs.