1. Mostly working, but still under test; usu. used with “in”: in beta. In the Real World, hardware or software systems often go through two stages of release testing: Alpha (in-house) and Beta (out-house?). Beta releases are generally made to a group of lucky (or unlucky) trusted customers.
2. Anything that is new and experimental. “His girlfriend is in beta” means that he is still testing for compatibility and reserving judgment.
3. Flaky; dubious; suspect (since beta software is notoriously buggy).
Historical note: More formally, to beta-test is to test a pre-release (potentially unreliable) version of a piece of software by making it available to selected (or self-selected) customers and users. This 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, module, 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, and the D test was the C test repeated after the model had been in production a while.