High blood pressure that doesn't have a known cause is called essential or primary hypertension. In contrast, secondary hypertension has a known cause. The difference between hypertension and primary hypertension is not distinct; all cases of high blood pressure are classified as either primary (essential) or secondary based on the presence or absence of an identifiable cause. Primary hypertension does not have a definitive cause, while secondary hypertension has a known cause. Both primary and secondary hypertension result in high blood pressure.