Normal Sinus Rhythm
Sinus rhythm (a.k.a. normal sinus rhythm) refers to the normal heart beat originating from the sinoatrial node. This is manifested as an upright P wave in lead II of the ECG.
Sinus bradycardia occurs when the heart rate is less than 60 beats per minute and sinus tachycardia when the heart rates is greater than 100 beats per minute in the setting of a sinus P wave on the ECG.
Note that ectopic atrial rhythms can have a P wave before each QRS complex, however the P wave is not upright in lead II since it originates in the atrium at a different location than the sinus node.