From The Art and Popular Culture Encyclopedia
A fanum is a plot of consecrated ground, a sanctuary, and from that a temple or shrine built there. A fanum may be a traditional sacred space such as the grove (lucus) of Diana Nemorensis, or a sacred space or structure for non-Roman religions, such as an Iseum or Mithraeum. Cognates such as Oscan fíísnú, Umbrian fesnaf-e, By the Augustan period, fanum, aedes, templum, and delubrum are scarcely distinguishable in meaning,
The fanum or ambulatory temple of Roman Gaul was often built over an originally Celtic religious site. The characteristic masonry structure had a central space (cella) and a peripheral gallery structure, both square.
The English word "profane" ultimately derives from Latin pro fano, "before, i.e. outside, the temple", "In front of the sanctuary," hence not within sacred ground.