a ha-ha is kind of landscaping fence, a combination of wall and ditch to keep animals from wandering onto the lawn. it's also the title of a very smart book of poetry by David Kirby--and it's through his poems that i first learned that the ha-ha wasn't a joke but a landscaping device. it's fun just to type the thing...ha-ha...apparently they became popular in the 1700s.
i'd never seen one until i walked by the Scottish palace of Holyrood the other day:
technically, this ha-ha is keeping the unsightly cars and parking strip out of sight from the palace, so that the view can roll towards Arthur's Seat without interruption. odd that this isn't used more often!