Definition from Wiktionary, the free dictionary
ground swell (plural ground swells)
- (surfing) waves generated by winds a long way away, possibly arriving at shore without local winds.
- 2005: Year-round the powerful ground swells generated in the notorious Roaring Forties 1000 kilometres to the south of Cape Leeuwin march north-east and meet with the reefs and beaches of the region. — MargaretRiver.com