In human anatomy, the thumb is the first digit on a hand. The human thumb is fully opposable to the tips of the other fingers in that it may position itself, and be folded inward, toward the rest of the hand and fingers, if so required. It rotates at the carpometacarpal joint and so can complete the sometimes quite delicate task of grasping objects by pressing them against the rest of the hand or finger(s).
Typical interdigital grips include the tips of thumb and second finger (forefinger/index finger) holding a pill or other small item, or thumb and sides of second and third fingers holding a pen or pencil.
The opposable or prehensile thumb is usually associated with the evolution of homo habilis, the forerunner of homo sapiens (the human being of today). This, however, is the result of evolution from homo erectus (around 1 million years ago) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.
The most important factor leading to the habile hand (and its thumb) is the freeing of the hands from their walking requirements - still so crucial for apes today, which in its turn was one of the consequences of the gradual pithecanthropoid and anthropoid adoption of the erect bipedal walking gait - and the simultaneous development of a larger anthropoid brain in the later stages.
Many animals, primates and others, also have some kind of opposable thumb or toe:
Grips
Evolution
Other animals with opposable thumbs or digits