Ha-ha is mentioned in chapters 9 and 10 of Mansfield Park.
It is a ditch, or, rather, a sunken fence by which means the formal garden of a (usually) grand house is separated invisibly from the park where livestock (cattle, deer and sheep) graze. One of its benefits is that from the direction of the house you can barely notice the break in the landscape. The other is to keep cows and deer in the park and not in the formal gardens thereby wreaking havoc eating the plants.
Before the introduction of the ha-ha, the only method of keeping livestock in the park separate from the gardens was by visually intrusive means of control , that is , fences and walls.
Horace Walpole in his essay On Modern Gardening( 1770) attributed the introduction of such garden features to the English landscape to the famous gardener, Charles Bridgman, partner of the equally famous Henry Wise:
But the capital stroke, the leading step to all that followed was (I believe the first though was Bridgman’s) the destruction of walls for boundaries, and the invention of fosses- an attempt then deemed so astonishing , that the common people called them Ha! Has! to express their surprise at finding a sudden and unperceived check to their walk.
One of the first gardens planted in this simple though still formal style was my father’s at Houghton. It was laid out by Mr Eyre, an imitator of Bridgman…
I call a sunk fence the leading step for these reasons. No sooner was this simple enchantment made, than levelling, mowing and rolling followed. The contiguous ground of the park without the sunk fence was to be harmonised with the lawn within; and the garden in its turn was to be set free from its prim regularity, that it might assort with the wilder country without. The sunk fence ascertained the specific garden, but that it might not draw too obvious a line of distinction between the neat and the rude, the contiguous outlying parts came to be included in a kind of general design: and when nature was taken into the plan, under improvements, every step that was made pointed out new beauties and inspired new ideas….
By adding an ha-ha to the landscape, the eye could rove freely beyond the immediate garden to the park onto the surrounding countryside.
Here are some pictures of the ha-ha at Burghley Park in Lincolnshire, which show the construction of the retaining wall. The last picture illustrates exactly why one needs an ha-ha : that deer has an evil intent in his eye….