[Latin: ‘island’]. In Roman architecture, either (1) an area of a town, typically enclosed by four streets, and probably corresponding to a smaller subdivision on the familiar CARDO/decumanus grid; or (2) a large ‘tenement’-type house, or apartment block, as well-illustrated at Roman Ostia. An insula can rise to four storeys, and we learn that the maximum permitted height (probably stipulated because of abuse) was 15 metres. The ground floor was frequently given over to shops. Balconies are quite common, and the number of the apartment is given on the staircase leading to it. Conditions in this type of building, especially at Rome, were often overcrowded, insanitary and disease-ridden.
The Macmillan dictionary of archaeology, Ruth D. Whitehouse, 1983