ha-ha

A ha-ha is a turfed ditch with one sloped side, and one vertical side that is faced with a masonry retaining wall. A ha-ha is a feature used in landscape garden design to keep grazing livestock out of a garden while providing an uninterrupted view from within.
Before mechanical lawnmowers, a common way to keep large areas of grassland trimmed was to allow livestock, usually sheep, to graze the grass. A ha-ha prevented grazing animals on large estates from gaining access to the lawn and gardens adjoining the house, giving a continuous vista to create the illusion that the garden and landscape were one and undivided. The name "ha-ha" was given to the feature because, when walking towards it from the garden, it would only become apparent to the observer when close to it.