ABMS Board Eligibility Periods and Transition Dates." Most physicians have between 5 to 7 years to remain "board eligible" before they must either: 1) become board certified, or 2) start over. I'm over-simplifying, but gone are the days of being grandfathered into a "board eligible" status (BE or board eligible was never formally recognized by the ABMS until now).
When you see a job advertisement that specifies that a physician must be BE or BC, you'll know what they're talking about:
BE = board eligible
BC = board certified
The ABMS Board Eligibility Policy can be viewed here.