very interesting question! i really liked this one :D

a better way to phrase the question would be:

What is the dot on the letter "I" called?

either way, the answer is:

a TITTLE

though, a tittle isn't necessarily only that dot, it could be a number of distinguishing marks on any number of letters, symbols, etc. but the dot is most definitely a tittle.

tittle.
(funny word)