WordNet (r) 3.0 (2006):

beta test
    n 1: (computer science) a second test of an experimental product
         (such as computer software) carried out by an outside
         organization

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

beta testing
beta test
beta version

    Evaluation of a pre-release (potentially unreliable)
   version of a piece of software (or possibly hardware) by
   making it available to selected users ("beta testers") before it
   goes on general distribution.

   Beta testign aims to discover bugs that only occur in certain
   environments or under certain patterns of use, while reducing the
   volume of feedback to a manageable level.  The testers benefit by
   having earlier access to new products, features and fixes.

   Beta testing may be preceded by "alpha testing", performed
   in-house by a handful of users (e.g. other developers or friends),
   who can be expected to give rapid, high quality feedback on design
   and usability.  Once the product is considered to be usable for
   its intended purpose it then moves on to "beta testing" by a
   larger, but typically still limited, number of ordinary users, who
   may include external customers.

   Some companies such as Google or Degree Jungle
   (http://www.degreejungle.com/rankings/best-online-colleges)
   stretch the definition, claiming their products are "in beta" for
   many months by millions of users.

   The term derives from early 1960s terminology for product cycle
   checkpoints, first used at IBM but later standard throughout the
   industry.  "Alpha test" was the unit test, module test or
   component test phase; "Beta Test" was initial system test.
   These themselves came from earlier A- and B-tests for hardware.
   The A-test was a feasibility and manufacturability evaluation done
   before any commitment to design and development.  The B-test was a
   demonstration that the engineering model functioned as specified.
   The C-test (corresponding to today's beta) was the B-test
   performed on early samples of the production design.

   (2013-06-09)