Isca is derived from a Brythonic Celtic word for flowing water, which was given to the Exe as well as to the River Usk (Welsh: Afon Wysg) in South Wales, on which Caerleon (known to the Romans as Isca Augusta) stands.
The south eastern boundary of the parish follows the route of the Fosse Way a Roman road that linked Exeter (Isca Dumnoniorum) in South West England to Lincoln (Lindum Colonia) in the East Midlands, via Ilchester (Lindinis), Bath (Aquae Sulis), Cirencester (Corinium) and Leicester (Ratae Corieltauvorum).
The town grew along the line of the Fosse Way, the ancient Roman road linking Exeter (Isca Dumnoniorum) to Lincoln (Lindum) on which it was an important stopping point.