A ditch with one side being a retaining wall; used to divide lands without defacing the landscape WordNet 3.0 & Open English WordNet

A ha-ha, also known as a sunk fence, blind fence, ditch and fence, deer wall, or foss, is a recessed landscape design element that creates a vertical barrier while preserving an uninterrupted view of the landscape beyond from the other side. Wikipedia

A recessed landscape barrier Wikipedia Disambiguation

A ditch with one vertical side, acting as a sunken fence, designed to block the entry of animals into lawns and parks without breaking sightlines. Wiktionary

Ditch acting as a sunken fence. Wiktionary (translation)