This is a triangulation station, otherwise known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station or trig beacon, but most commonly called a trig point. In the UK, they are generally tapering concrete pillars about four feet high with an inscribed bench mark identifying their location.
All trig points occupy high land relative to their location, but not all are on the summit of a hill because the most important factor is the line of sight, rather than the height.
Trig points were erected to enable geodetic surveying; sort of the science of measuring the landscape in 3D. To enable this to happen, most trig points have a brass plate set into the top of them. This has three equally spaced branches radiating from a central point. I'd always assumed that this was a kind of compass pointer to measure things on the horizon, but it isn't. It's the mounting for a theodolite; a precision instrument for measuring horizontal and vertical angles (a bit like having a compass built into a telescope)
Trig points were first erected in 1935 by the Ordnance Survey and were so positioned that, on a clear day, it should be possible to stand by one trig point and be able to see at least two more. In this way, the UK was covered in a grid of triangles which could be used to measure the country with a high degree of accuracy. It is thanks to trig points and this grid that we have Ordnance Survey maps. I LOVE Ordnance Survey maps :)
When the grid was first completed, there were 6557 trigs in the UK. Because their map-making usefulness has been superceded by satellites and stuff, many have now been removed, but they are still very useful to hill walkers. In fact, there are those who 'collect' trig points; a practice known as Trig Point Bagging. It's a bit like train spotting except that you go to the trig point rather than waiting for the train to come to you. It would be an enormous job to bag every trig point, so they are usually done by area or category - maybe all of the trig points within a particular National Park, or over a certain altitude. For example, there are 1813 trig points over 300m, but only 676 over 500m and a mere 37 over 1000m. Hmmm. I think I'll set that as a challenge for Mark. Go bag every trig point over 1,000m. Right up his street!
I have no idea how many trig points I have visited during my lifetime. Quite a few! I suppose it's a bit late to start counting now... but whenever I do walk past another one, I will try to remember to take a photo and post it on here.