According to Wikipedia, the word 'Yat' is an Old English word for a gate, and it refers to the gorge that the river has made at this point. (Nobody really knows who Symonds was, but the yat has been named after him or her since at least 1665.)
The famous viewpoint on the eastern side of the gorge (which I'd always thought was the Yat) is more properly known, apparently, as Symonds Yat Rock. It's this prominence that forces the river Wye to take a detour northwards, heading south after about 2.5 km (1.6 miles) to pass it on the other side.
There is also a village near the viewpoint, which the river flows through (on its way back south), dividing it into Symonds Yat East and Symonds Yat West.
(Wikipedia also refers to something nearby called Seven Sisters Rocks, which may or may not be all or part of Symonds Yat Rock.)
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