Definition - What does Sinclair ZX81 mean?
The Sinclair ZX81 is a personal computer developed in Britain and released in 1981 that is known for introducing the personal computer into the mainstream. Much of its popularity is attributed to its affordability, as its predecessor (the ZX80) was much more expensive. The ZX81 also became known for having many problems, however, such as a tendency to crash frequently.
Techopedia explains Sinclair ZX81
In the early 1980s, personal computers were just starting to make their way into the home. The ZX81 was the successor of the ZX80, upon which it made a number of improvements. Millions of were sold before the model was discontinued. In 1982 it was rebranded the Timex-Sinclair 1000 for U.S. markets.
The ZX81 featured an extension port on its back, however the connection had a tendency to fail, which would cause the system to crash. The machine ran in "slow" and "fast" modes, which primarily affected screen refresh rates ("slow" constantly refreshed, while "fast" only refreshed when computations were complete), so that users had the choice between better graphics or better performance.