A right bundle branch block (RBBB) is a heart block in the right bundle branch of the electrical conduction system.During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bundle branch. The left ventricle, however, is still normally activated by the left bundle branch. These impulses are then able to travel through the myocardium of the left ventricle to the right ventricle and depolarize the right ventricle this way. As conduction through the myocardium is slower than conduction through the Bundle of His-Purkinje fibres, the QRS complex is seen to be widened. The QRS complex often shows an extra deflection that reflects the rapid depolarisation of the left ventricle followed by the slower depolarisation of the right ventricle. Incomplete right bundle branch block
Incomplete right bundle branch block (IRBBB) is an conduction abnormality in the right bundle branch block. While a complete RBBB has a QRS duration of 120 ms or more, an incomplete block has a wave duration between 110 and 120 ms. It has a relatively high prevalence, a study conducted on young Swiss military conscripts with a mean age of 19 years found a prevalence of 13.5%. It affects patients of all ages, more commonly males and athletes, however is not always a benign finding. Therefore, if abnormalities are detected on the physical exam, further testing should be done to exclude heart disease.There is no consensus in the literature regarding criteria for diagnosis.
However, according to the American Heart Association/American College of Cardiology Foundation/Heart Rhythm Foundation (AHA/ACCF/HRS) it is defined by the following finding in adults:

QRS wave duration between 110 and 120 ms.
rsr, rsR, or rSR in leads V1 or V2. S wave of longer duration than R wave or greater than 40 ms in leads I and V6. Normal R wave peak time in both V5 and V6, but greater than 50 ms in V1.The first three criteria are needed for diagnosis. The fourth is needed when a pure dominant R waver is present on V1. Causes
Common causes include: normal variation, changes in bundle branch structure - such as mechanical stretching, chest trauma, right ventricular hypertrophy or strain, congenital heart disease such as atrial septal defect, and ischemic heart disease. In addition, a right bundle branch block may also result from Brugada syndrome, pulmonary embolism,  rheumatic heart disease, myocarditis, cardiomyopathy, or hypertension.Causes for incomplete right bundle branch block are exercise-induced right ventricular remodeling, increased RV free wall thickness, especially in athletes due to prolonged endurance exercise. Diagnosis
The criteria to diagnose a right bundle branch block on the electrocardiogram:

The heart rhythm must originate above the ventricles (i.e., sinoatrial node, atria or atrioventricular node) to activate the conduction system at the correct point. The QRS duration must be more than 100 ms (incomplete block) or more than 120 ms (complete block). There should be a terminal R wave in lead V1 (often called "R prime," and denoted by R, rR, rsR, rSR, or qR).