g-2. g minus 2. g is the magnetic moment of an electron. It is expected to be very close to 2. g minus 2 is a value that can be measured, and that can be calculated, both very precisely.

If I understand the current situation, for electrons g-2 agrees between experiment and measurement to 10 digits. For muons, though, it doesn't. (Muons are harder to measure, because they decay. And they are somewhat less well understood theoretically, so there's room on both sides of that question.)