noun

• (obsolete) Any snake.
• A name loosely applied to various snakes more or less resembling the viper.
• (chiefly Britain) A small venomous serpent of the genus Vipera
• The common European adder (Vipera berus).
• The puff adders, of Africa (genus Bitis).
• (US, Canada) Any of several small nonvenomous snakes resembling adders
• Lampropeltis triangulum (milk snake).
• Heterodon spp. (hog-nosed snakes), a genus of harmless colubrid snakes found in North America
• Certain venomous snakes resembling other adders
• Acanthophis spp. (death adders), elapid snakes found in Southeast Asia and Australia
• Agkistrodon contortrix mokasen, the northern copperhead, a venomous viper found in the eastern United States
• A sea stickleback or adder fish (Spinachia spinachia).
• Someone who or something which performs arithmetic addition; a machine for adding numbers.
• An electronic device that adds voltages, currents or frequencies.
• Something which adds or increases.
• (informal) A person who has attention deficit disorder.