Documents probably from the fourth century A.D., known as the Regionaries, list contemporary landmarks of the city of Rome. The regions of the title are the 14 areas into which Emperor Augustus divided Rome. According to the Regionaries, there were 44,850 insulae and 1781 domus. Generally, insula (literally, 'island') is translated as apartment building or block and domus as house/home, although here you might think "mansion". The individual apartments in which people lived are also sometimes called insulae, but the term cenaculum seems more common, at least by the late imperial period. An insula might also refer to a taberna (shop) or a floor in an apartment building. An insula is generally believed to be an independent building.
The significance of the 44,850 insulae in the Regionaries is that through them it might be possible to estimate the population of Rome in the Imperial period. Unfortunately, it is not yet possible to decide conclusively what constitutes even an insula, let alone what a suitable number of occupants might be in each, and so it remains impossible to deduce how large the population of Rome was.