Bradycardia, as others pointed out above, is a heart rate below 60bpm. It is so named because it varies significantly from the norm, and specific symptoms are associated with it.
My HR is in the 50s mostly, without problems. That seems to be the case with many people with HRs in the 50s.
People who are highly atheletic (MarkU) and those who are slight of frame may have less trouble with a resting heartbeat in the 40s, but most people will experience difficulties and symptoms from it. As it can affect consciousness or cause visual disturbances, it's a danger to most of those with that issue. As seen above, it's sufficient cause in most people for reassessment of their prescriptions or even a pacemaker.
While people may feel that the low pulse isn't an issue for them, the famous incident when George Bush choked on a pretzel was partly blamed by some White House physicians on his low, atheletic resting pulse, apparently listed as being in the high thirties to mid forties, which may have been a cause for him fainting.
Most people should be looking to their physicians to either rework their prescriptions to bring their pulses up from the forties (if you are taking BP meds, the pulse in the 40s is not likely to be all your body's idea), determine if there is some other action that needs to be taken to bring the heart rate up, or determine that it's a safe rate for them as an individual.
Best wishes,