What is "aesica"

Aesica (with the modern name of Great Chesters) was a Roman fort, one and a half miles north of the small town of Haltwhistle in Northumberland. It was the ninth fort on Hadrian's Wall, between Vercovicium (Housesteads) to the east and Magnis (Carvoran) to the west. Its purpose was to guard the Caw Gap, where the Haltwhistle Burn crosses the Wall. The B6318 Military Road passes about half a mile to the south of the fort.