The thumb is the first digit of the hand. 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 noun for thumb is pollex (compare hallux for big toe), and the corresponding adjective for thumb is pollical.
The English word "finger" has two senses, even in the context of appendages of a single typical human hand:
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:
The thumb contrasts with each of the other four by being the only digit that:
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.