OCD4 s.v. pomerium: 'explained in antiquity as meaning what comes after, or before, the wall -- was the line demarcating an augurally constituted city'.
For the word in Latin authors and inscriptions, see generally Lewis & Short s.v. In this Greek transliterated form it occurs in Plutarch (Romulus 11.4) and frequently in Cassius Dio. It is glossed in ps.-Zonaras as to\ tou= tei/xous kallw/pisma "the decoration of the wall"; the same lexicon has an entry for pomh/rion with omicron, glossed as to/pos "a place."