A henge is a roughly circular or oval-shaped flat area over 20m in diameter which is enclosed and delimited by a boundary earthwork that usually comprises a ditch with an external bank. Access to the interior is obtained by way of one, two, or four entrances through the earthwork. Internal components may include portal settings, timber circles, post rings, stone circles, four-stone settings, monoliths, standing posts, pits, coves, post alignments, stone alignments, burials, central mounds, and stakeholes.
Henges are variously identified through fieldwork as earthworks or as cropmarks revealed by aerial photography. In both cases it is the size and arrangement of the earthworks that allows identification.
Henges can sometimes be confused with other kinds of circular monuments, especially when levelled by ploughing. The main sources of confusion are with: ring cairns, enclosed cemeteries, fancy barrows, various classes of later prehistoric enclosed settlements, Roman signal stations, Roman amphitheatres, and windmill mounds. In all cases close attention to the position and extent of the bank and the entranceways is important when distinguishing henges from these other classes. Care must also be taken not to confuse henges with sites where some of the components of henges exist as single monuments in their own right, for example timber circles and stone circles.
Specifically excluded from this definition of henges are the small henge-like monuments with an internal diameter of less than 20m which are defined for the purposes of the Monuments Protection Progarmme as hengi-form monuments, and the very large henge-like enclosures with an irregular outline which are defined as henge enclosures. Also excluded here are sites sometimes referred to as henges but which have a bank inside the ditch.
Henges are generally of late Neolithic date and are currently interpreted as ritual or ceremonial monuments, perhaps meeting places, trading centres, or sacred areas of some sort.