Is a thumb also a finger?

The thumb has a different name compared to the other fingers (index, middle, ring, little) and it does not end with "finger".

Also, when referring to the hand, I have seen literature where it is described as "5 digits on each hand" and that is probably more accurate.

In light of that,

• Is saying that I have "5 fingers on my left hand" technically just wrong?
• Or is the thumb a "special" type of finger and so it is acceptable to treat it as a finger in general?

Are there any rules/guidelines related to this in English that anyone can throw light on or is it that I am just nitpicking?

The thumb can be treated as a finger but mostly within the context of fingers. Both digits and fingers are hypernyms of the word thumb. Here are my dictionary* definitions:

• thumb

  the short, thick first digit of the human hand, set lower and apart from the other four and opposable to them.

• finger

  each of the four slender jointed parts attached to either hand (or five, if the thumb is included)

• digit

  a finger (including the thumb) or toe

Thus, saying you have 5 fingers on [your] left hand is not wrong but some may consider it awkward, since there is really nothing special about having five digits on either hand! Indeed, it is acceptable to treat the thumb as a finger, as I earlier alluded to, and I give a few examples to illustrate:

• Count on your fingers.
• He rubbed his fingers vigorously.
• Show me your fingers!
• Your fingernails are dirty.
• All my fingers are cold.
• How many fingers am I holding up on my hands?
• 'Which finger did it bite?' 'It bit my THUMB!'
• Her fingers flew nimbly across the keyboard.
• Have a look at the fingering chart.
• He fingered the steaming mug gingerly, taking quick sips in between mouthfuls of bread.

However, one would rarely, if ever, hear finger used in a singular sense to directly mean thumb. Digit[s] is rarely used in everyday conversation, and since it also includes toes, one is probably most likely going to hear/use fingers when the manual digits are involved.

*New Oxford American Dictionary, 2nd Edition

Etymologically, the words finger, fist and five stem from the same Indo-European roots. It makes sense to consider there to be five fingers on each hand. Instead of relying on definitional nuances, etymology can provide a clearer and less divisive answer for this. Even tie meanings change, they wouldn't randomly jump around. Etymological changes in meaning are intuitive and understandable, like Latin spiro, meaning "I breathe", becoming the word 'spirit'.