A finger is a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates. Normally humans have five digits, termed phalanges, on each hand (exceptions are polydactyly, hypodactyly and digit loss). The first digit is the thumb, followed by index finger, middle finger, ring finger, and little finger or pinky. Some other languages use the same generic term for all five digits of a hand.
English dictionaries describe finger as meaning either one of the five digits including the thumb, or one of the four excluding the thumb (in which case they are numbered from 1 to 4 starting with the index finger closest to the thumb). Linguistically, it appears that the original sense was to include the thumb as a finger: *penkwe-ros (also rendered as *penqrós) was, in the inferred Proto-Indo-European language, a suffixed form of *penkwe (or *penqe), "five", which has given rise to many Indo-European-family words (tens of them defined in English dictionaries) that involve or flow from concepts of fiveness.
Chimpanzees have lower limbs that are specialized for manipulation, and (arguably) have fingers on their lower limbs as well. The term 'finger' is not applied to the digits of most other animals, such as canines, felines, or ungulates, none of which can engage in fine manipulation with their forelimbs as a primate can.
Each finger may flex and extend, abduct and adduct, and so also circumduct. Flexion is by far the strongest movement. In humans, there are two large muscles that produce flexion of each finger, and additional muscles that augment the movement. Each finger may move independently of the others, though the muscle bulks that move each finger may be partly blended, and the tendons may be attached to each other by a net of fibrous tissue, preventing completely free movement. This is particularly noticeable when trying to extend the fourth digit (third finger) with the others flexed.
Fingers are usually moved under conscious control. In humans, they are used for grasping, typing, grooming, writing, caressing, and many other activities. They are also used in signaling, as when wearing a wedding ring, finger counting or when communicating in sign language.
Aside from the genitals, the fingertips possess the highest concentration of touch receptors and thermoreceptors among all areas of the human skin, making them extremely sensitive to heat (and cold), pressure, vibration, texture, and moisture. Thus fingers are commonly used as sensory probes to ascertain properties of objects encountered in the world, and so they are prone to injury.
Fingers do not contain muscles other than arrector pili muscles. The muscles that move the finger joints are in the palm and forearm. The long tendons that deliver motion from the forearm muscles may be observed to move under the skin at the wrist and on the back of the hand.
The fingers are commonly affected by diseases such as rheumatoid arthritis and gout. Diabetics often use the fingers to obtain blood samples for regular blood sugar testing. Raynaud's phenomenon is a neurovascular disorder that affects the fingers.