The Free On-line Dictionary of Computing (30 December 2018):

ZX-81

    An even more successful version of the Sinclair
   ZX-80, featuring a large uncommitted logic array instead
   of much discrete logic, an improved BASIC, and rather more
   expandability (it could take 16kb RAM packs).  It was
   launched around 1981 and was eventually replaced by the
   Spectrum.

   (1995-11-04)