If I remember my Vos Savant aright the dot over the i is but one version of a tittle. Using Jackie's analogy, the tittle is the whisky. The question was "If the double dots are called a dieresis, what is the single dot over an i called?" Marilyn said it was a tittle and said the the dieresis was also a tittle, as were accents of both stripes, macrons, and circumflexes. She did not offer a term unique to the dot over the i.

Edit: Thanks again to Dr Bill for catching, in this case, a flat out misspelling. Can't plead typo on this one. It's dieresis or diæresis, not diaresis.