Public architecture is the word archaeologists use to refer to buildings (or empty spaces such as plazas) which are constructed for public purposes. Buildings in cities and towns of the human past had many uses--homes, of course, but also as temples and meeting places and cemeteries and other communal reasons.
Evidence for a building having been for a 'public use' might include suggestions that it was a community effort: a comparatively large size over all, elaborate decorations, number of rooms, etc. Also considered evidence is the lack of the kind of domesticate rubbish that you see at a house or cooking area, including food rubbish.
As comparison, see
This glossary entry is a part of the About.com Guide to the Characteristics of Ancient Civilizations, and part of the Dictionary of Archaeology.
