Sinclair Research was a British electronics company founded by inventor Sir Clive Sinclair in the 70s. It is best known for its ZX Spectrum computer, hugely successful due to its very low price.
The ZX would be their last success, however: their finances were ruined by a number of overambitious projects (such as artificial intelligence and wafer-scale integration) and poorly received products (especially the ZX's ill-fated successor, the Sinclair QL, along with the notoriously unreliable ZX Microdrive - but also the C5 electric car and the TV80 pocket television). The company was then bought by long-time rival Amstrad, who continued to produce the Spectrum line until the early 90s.
As a sidenote, a group of former Sinclair engineers formed Flare Technologies, who designed an advanced arcade chipset that would be used in the never-released Konix Multisystem console, and another that served as the basis of the Atari Jaguar.