The dot over the lowercase "i" is called a tittle. It is also seen in the lowercase "j". The tittle first appeared in Latin manuscripts in the 11th century, to distinguish the letter i from strokes of nearby letters. Although originally a larger mark, it was reduced to a dot when Roman-style typefaces were introduced.
The word "tittle" is rarely used. Its most prominent occurrence is in the introduction to the Antithesis of the Law in the Gospel of Matthew (5:18): "For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled" (NKJV). The quotation uses them as an example of extremely minor details. The phrase "jot and tittle" indicates that every small detail has received attention.