A long mortuary enclosure is an oblong-shaped area of land up to 150m in length, bounded by narrow, fairly straight ditches on all sides, with slightly rounded corners, and containing an open space edged by a perimeter bank set just inside the ditch. Characteristically, there are two or more major causeways across the ditch which served as entrances. The function of long mortuary enclosures is not known with certainty; they are generally interpreted as ceremonial monuments of early and middle Neolithic date.
Long mortuary enclosures can sometimes be confused with the quarry ditches and/or palisade trenches found in some types of long barrow. Regarding the former, it is generally accepted that the ditches of long mortuary enclosures are much smaller than the quarry ditches of long barrows, while in the latter there is no evidence that the ditches of long mortuary enclosures ever held palisades or fences. The ditches of oval barrows are potentially more confusing, and some oval barrows have in the past been published as long mortuary enclosures. The larger size, slightly squared terminals, and straight sides of long mortuary enclosures usually allows them to be distinguished from oval barrows, although in the case of sites only identified from cropmarks these two classes of monument can be difficult to separate. Another possible source of confusion is that long mortuary enclosures have the same general ground-plan, and are about the same size, as the rectangular Celtic sanctuaries known as viereckschanze. However, until more is known of such sites in Britain little guidance can be given as to how the two classes may be distinguished on the basis of field evidence alone. Loveday and Petchey (1982, 23-4) suggest that, as a general rule, oblong enclosures which are spatially associated with other distinctive early prehistoric ritual sites are more likely to be long mortuary enclosures than later prehistoric sanctuaries.
Specifically excluded from the class of long mortuary enclosures are various small square and circular features forming components of long barrows, bowl barrows, oval barrows, and D-shaped barrows, which are sometimes loosely termed mortuary enclosures, and also the rather larger rectangular enclosures over 250m long which are defined as cursus monuments.