Is there any historical consensus on what stone henge actually is?

What do you mean 'what it is'? It's clear what it 'is' in one sense; it's a big pile of stones. This pile of stones has been constructed a very long time ago, and could have had a wide range of meanings for the different generations in whos world this monument stood. This meaning (what is stonehenge to them?) is much more interesting than the factual, objective 'what it is'. A nice outline of meaning vs. the thing can be found in Tilley's discussion of materials and materiality (materiality of stone, 2004). The idea of 'multiple sites' (as in, a single archaeological location with many attributed meanings) can also be found with Roundtree 2001, The past is a foreigner's country, in Journal of Contemporary Religion 16, about the significance and meaning of Maltese temples to different people, and in Bartu 2000, Where is Catalhoyuk, in Ian Hodder's Towards Reflexive Method in Archaeology: An Example from Catalhoyuk, about the construction of the meaning of Catalhoyuk both in the field, in the museum, and in the minds of people.

I assume you mean what it was used for, or for what function it was built (like today we build buildings for offices, or as houses for living in, or roads for driving on). The thing with Stonehenge is that it has a long use/construction history; the current form of the monument is only the end result of thousands of years of refurbishment/reconstruction. Currently it is a British tourist attraction, for which it was restored in the beginning of the last century. It had been regarded as a curiosity for millennia before then, with some Bronze- and Iron Age re-use and featuring in Medieval mythology as well (as outlined in Parker Pearson ****). It is quite likely that it was already regarded as an ancient (ancestral?) monument during the Neolithic too, during the later re-building phases when the lintel stones were added for example. The Henge-type model is, however, much more traditional and has some significance regarding calenders and by extension, astronomy, and the dead. It is quite likely that after its construction, rituals were performed in or near the site, possibly also in relation to either calendrical/astronomical events or the dead. This does not mean it was a 'temple' or a 'cemetery' in our senses of the word, were gods were venerated or the dead disposed of; these rituals could be of an entirely different character, like how non-western communities performed calendrical or mortuary rituals in historic times as well. Yet this does not automatically mean that Stonehenge was built to make a place to facilitate these rituals, like how we build a road for driving on. It could also be that the construction of Stonehenge in itself is more important than its eventual use; the construction process might have taken multiple decades, and the construction sequence definitely took up multiple centuries (with its constant rebuilding phases). This idea of a neolithic megalithic monument collecting multiple meanings can also be found in Cummings' 2008 article 'The architecture of monuments', in Pollards 'Prehistoric Britain'. Richard Bradly also explored the different roles of monuments in shaping worldview of British populations from the Mesolithic to the Bronze Age, showing how monuments can be adopted to reflect changing worldviews in his 1998 book 'The significance of monuments'.

To me this also suggests that the construction might have been more important than its use, comparable to how we today celebrate the construction of the Burj Al-Khalifa as the highest building, while it's function as an office building is, in fact, secondary. The same with the pyramids: ancient Egyptian theology did not require such a huge building for the correct process of the pharao to the afterlife, but the construction of the pyramid during the pharao's life was in itself a social statement. The focus on the construction, rather than the use phase of monuments is rather old, and already was a major point in Renfrew's interpretation of Wessex megalithic monuments in the 1973 article 'Monuments, mobilisation and social organisation in Neolithic Wessex'. While I would not use his materialist/Marxist interpretation regarding social organisation without recent stress on meaning of postprocessual archaeology, his starting point is still quite valid and a nice counterpoint to Parker Pearsons more ephemeral views.

Edit: people apparantly really dislike blocks of text without references. They like paragraphs and peer-review, so edited accordingly.