The thumb is the first digit
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....

 of the hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

. When a person is standing in the medical anatomical position (where the palm is facing anteriorly), the thumb is the lateral-most digit. The Medical Latin English adjective for thumb is pollical.
The English word "finger" has two senses, even in the context of appendages of a single typical human hand:
  1. Any of the five digits.
  2. Any of the five terminal members of the hand, especially those other than the thumb.

Linguistically, it appears that the original sense was the broader of these two: penkwe-ros (also rendered as penqrós) was, in the inferred Proto-Indo-European language
Proto-Indo-European language
The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

, a suffixed form of penkwe (or penqe), which has given rise to many Indo-European
Indo-European languages
The Indo-European languages are a family of several hundred related languages and dialects, including most major current languages of Europe, the Iranian plateau, and South Asia and also historically predominant in Anatolia...

-family words (tens of them defined in English dictionaries) that involve or flow from concepts of fiveness.

The thumb shares the following with each of the other four fingers:
  • Having a skeleton of phalanges, joined by hinge-like joints that provide flexion toward the palm of the hand
  • Having a "back" surface that features hair and a nail, and a hairless palm-of-the-hand side with fingerprint
    A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...

     ridges instead

The thumb contrasts with each of the other four by being the only digit that:
  • Is opposable to the other four fingers
  • Has two phalanges rather than three
  • Has greater breadth in the distal phalanx than in the proximal phalanx
  • Is attached to such a mobile metacarpus (which produces most of the opposability)

and hence the etymology of the word: "tum" is Proto-Indo-European for " swelling " (cf "tumour" and "thigh") since the thumb is the stoutest of the digits.
In humans, opposition and apposition are two movements unique to the thumb but these words are not synonyms:

Primatologists and hand research pioneers J. Napier
John Napier (primatologist)
John Russell Napier, MRCS, LRCP, D.Sc. was a British primatologist, paleoathropologist, and physician, who is notable for his work with Homo habilis and OH 7, as well as on human and primate hands/feet...

 and P. Napier
Prudence Hero Napier
Prudence Hero Napier was one of Britain's most eminent primatologists, and the world's leading expert on the taxonomy of primates. She was the widow of the primatologist, John Napier. -Biography:...

 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
Carpometacarpal joint
The carpometacarpal joints are five joints in the wrist that articulates the distal row of carpal bones and the proximal bases of the five metacarpal bones....