Shorthand for "eqivalent to the HaltingProblem".  In other words, not decideable (in finite time on a TuringMachine or a TuringEquivalent device) for all possible inputs.

Note that many HaltingEquivalent problems ''are'' decideable for a subset of their inputs; in many case the subset is a useful one.  Also note that being decideable does not necessarily mean  tractable; computation of the AckermannFunction is decideable for all values, but you'll need a rather large and fast computer to compute A(4,4)...