pomerium
The ritually established and sacred formal boundary of the territory of a Roman city; the territory thus bound.
historical
Nay Sidonius Apollinaris aſſures us farther, that the Place where St. Peter was buried, tho' there was then a Church built over it, was ſtill in his Time, An. 470. without the Pomœria, or Space before the Walls of Rome. For ſpeaking of his Journey to Rome, he ſays, before ever he came at the Pomœria of the City, he went and ſaluted the Church of the Apoſtles, which ſtood in the Via Triumphalis.
Where it comes from
From Latin pōmoerium, pōmērium (“the religious boundary of a city”), either from post (“behind”) + moerus, mūrus (“wall”) + -ium (neuter form of -ius (adjective-forming suffix)), or derived from an Etruscan word.
Adapted from the Wiktionary entry pomerium (CC BY-SA 4.0).