What is the dot over a lowercase 'i' or 'j' called?
The word is rare but official.
The name 'tittle' comes from the Latin 'titulus', meaning an inscription or heading. In medieval Latin manuscripts the mark was used to make the letters i and j stand out in dense handwriting, where strokes of neighbouring letters like m, n and u were easy to confuse. Dictionary.com says this practice dates from about the 11th century.
Early on, the mark was a larger stroke. When Roman-style type spread in the late 15th century, it shrank to the small dot seen today. Because it is a separate mark from the letter body, it counts as a diacritic. In modern Turkish the dot matters: dotted 'i' and dotless 'ı' are separate letters representing different sounds, and each has its own capital form.
The word also lives on in idiom. The phrase 'to a tittle' means exactly or precisely, and dictionaries link it to the common phrase 'to a T'. An early example is cited from Edward Hall's Chronicles around 1548, and the word has long meant any tiny mark or detail. The dot over a letter is therefore both a practical scribal device and the root of a phrase for meticulousness. Be wary of claims about one inventor of the dot; it evolved gradually among scribes.