What is sinus bradycardia?

Slow heart rate. Generally a heart rate less than 50 beats per minute coming from the heart's natural pacemaker is considered sinus bradycardia. It is not necessarily abnormal as well conditioned athletes often have low heart rates. Some medications such as beta blockers slow the heart rate to this level.
Slow heart rate. A heart rate slower than expected for age when compared to a particular population that still originates from the normal region of the heart (sinus node) and travels down the normal electrical pathways of the heart. Brady in medicine refers to slow, and in this case cardia to heart rate.