An adder is a type of venomous snake found across Europe and parts of Asia. Adders are relatively small snakes, usually less than two feet long, with distinctive zigzag patterns running down their backs. In Britain, the adder is the only venomous snake, though its bite is rarely dangerous to humans if treated properly.

The name comes from an Old English word meaning “serpent.”

Adders hibernate during cold months and emerge in spring to sun themselves on rocks and logs. They're shy creatures that prefer to slither away rather than strike, biting only when threatened or accidentally stepped on. In medieval times, people feared adders greatly and told many exaggerated stories about their danger.

In mathematics and computing, an adder is also a circuit or device that adds numbers together. Computer engineers design adders to perform the basic arithmetic that makes calculators and computers work.