The thumb is the first digit (finger) of the hand. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb is "pollex" (compare "hallux" for big toe), and the corresponding adjective for thumb is pollical.

Most primates (humans, apes, and Old World monkeys) and some other animals have opposable thumbs. Humans can move their thumb farther across their hand than any other primate.

Having opposable thumbs helps in grasping things more easily, picking up small objects, and eating with one hand. The evolution of the fully opposable thumb is usually associated with "Homo habilis", a forerunner of "Homo sapiens".

Primatologists and hand research pioneers John and Prudence Napier defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with – or diametrically opposite to – the terminal pads of one or all of the remaining digits." For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint). Arguably, this definition was chosen to underline what is unique to the human thumb.

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