The thumb is the first 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.
Finger and thumb
The English word "finger" has two senses, even in the context of appendages of a single typical human hand:
Any of the five digits.
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, a suffixed form of penkwe (or penqe), 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.
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 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 fingers.