What does marked bradycardia mean?

HR < 50. Bradycardia is defined as heart rate less than 60 bpm. I don't know if there is a formal definition of severe bradycardia, but i would consider bradycardia severe if the heart rate was less than 50.
Slow heart rate. Slow heart rate less than 50 BPM is marked bradycardia. If it is sinus and a symptomatic then it is normal. However any other mechanism or symptoms call for an evaluation.