A hengi-form monument is a flat, roughly circular area of ground, typically between 5m and 20m in diameter, which is enclosed within a modest earthwork comprising a ditch with a bank on the outside. There may be either one entrance through the earthwork, or two opposing entrances. Pits, cremation pits, postholes, stone-sockets and graves may be present within the enclosure, and postholes and cremation pits may be present around the site.
Hengi-form monuments are usually recognized from aerial photography or excavation, occasionally from surface fieldwork. Excavation is usually required to authenticate hengi-form monuments as they are superficially similar in size and shape to many other classes of monument. The most common causes of confusion are with various types of bowl barrows, bell barrows, and fancy barrows, particularly heavily denuded examples. The ditch of a hengi-form monument can look very similar to some of the penannular mortuary enclosures surrounding the central burial under various classes of round barrow, the difference is that hengi-form monuments tend to be slightly larger and the earthworks more substantial. The outer ditches of various classes of round barrow can also be confused with the earthworks of a hengi-form monument, and here it is essential to examine carefully the number and nature of causeways and the position and extent of any banks; hengi-forms have fairly wide outer banks. Other sources of confusion include Roman signal stations, stone hut circles, windmill mounds, and the drainage gullies of later prehistoric round houses.
Specifically excluded from this definition of hengi-form monuments are henges proper, which generally have an internal diameter of over 20m, and are regarded as a separate class.
Hengi-form monuments are currently interpreted as ritual or ceremonial monuments of late Neolithic date, in most cases closely connected with burial.