This is some excellent research on the origin of the word henge done by David B.

Click on the photo to see a run of Paula B’s
terrific photos from the henge event.

The following is from the Online Etymology Dictionary. These references provide an historical perspective of the origin of the word.
henge  1740, noted as a Yorkshire word for structures such as Stonehenge.
hinge   c.1300, “the axis of the earth;” 1380 as “movable joint of a gate or door,” not found in O.E., cognate with M.Du. henghe “hook, handle,” M.L.G. henge “hinge;” all derived from the root of the verb hang on notion of the thing from which a door hangs.

Stonehenge
c.1130, Stanenges, lit. “stone gallows,” perhaps so called from fancied resemblance to old-style gallows with two posts, with the second element related to the verb hang. Some antiquarians suggest the notion may be of “supported in the air, that which hangs in the air” (cf. henge-clif, for L. præruptum), in ref. to the lintel stones, but the order of the elements and the inflexion is against this. An ancient name for it was the Giant’s Dance.

Below is the definition of a henge from http://www.orkneyjar.com/history/henge.htm, one of the links of the Orkney Islands’ web site. I recommend that you visit this site and click on the links for several henges in these islands.
A henge is a roughly circular or oval-shaped flat area enclosed and delimited by a boundary earthwork – usually a ditch with an external bank.
The most distinctive components of any henge monuments are its bank and ditch. Most henges have either a single ditch or a pair of concentric ditches surrounding the central area.
This is not always the case, however, with some henges having no ditch while others have three.
The soil and bedrock taken from the ditch was used to build the henge bank which generally lay outside the ditch. The sizes of the banks varied proportionally with the size of the ditches. Typically, however, they seem to have been fairly broad at the base, five metres to 30 metres wide and up to five metres high.
Access to the central area was via formal entrances through the earthwork. Most henges have either one entrance or two opposed entrances.
The alignment of henges seen in the position of their entrances is highly variable and may have been as much conditioned by local geography as by any preferred orientation. There is, however, a slight tendency for henges with a single entrance to have that entrance set in the north or north-east sector while sites with two entrances are aligned SE-SSE to NW-NNW or ENE-E to WSW-W.
The original purpose and function of henge monuments is not fully understood. Because of the arrangement of banks and ditches it is generally accepted that they are ceremonial or ritual monuments.
Henges are generally classified into four main types according to the number of entrances and ditches they have. The traditional classification is as follows:
Class I Single entrance, single bank, and, usually, a single ditch circuit.
Class IA Single entrance, single bank, and double circuit of ditches.
Class II Two opposed entrances, single bank, and single ditch circuit.
Class IIA Two opposed entrances, single bank, and two or more circuits of ditches.
Class III Four opposed entrances, single bank, and single ditch circuit.