[ad.der] n [ME, alter. (by false division of a naddre) of naddre, fr. OE naedre; akin to OHG natara adder, L natrix water snake] (14c) 1: the common venomous viper (Vipera berus) of Europe; broadly: a terrestrial viper (family Viperidae)
2: any of several No. American snakes (as the hognose snakes) that are harmless but are popularly believed to be venomous
[2]add.er n (1580): one that adds; esp: a device (as in a computer) that performs addition