Aulus Gellius’ Attic Nights, XIII. 14,1

The augurs of the Roman people who wrote books On the Auspices have defined the meaning of pomerium in the following terms:
“The pomerium is the space within the rural district designated by the augurs along the whole circuit of the city without the walls, marked off by fixed bounds and forming the limit of the city auspices.”
(That is to say, the pomerium separated the ager Romanus, or country district, from the city. The auspices could be taken only within the pomerium. When a furrow was drawn and the earth turned inward to mark the line of the city walls, the furrow represented the pomerium).
Now, the most ancient pomerium, which was established by Romulus, was bounded by the foot of the Palatine hill. But that pomerium, as the republic grew, was extended several times and included many lofty hills. Moreover, whoever had increased the domain of the Roman people by land taken from an enemy had the right to enlarge the pomerium.

Aulus Gellius’ Noctium Atticarum,XIII. 14,1