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? Hypertension is simply high blood pressure, while primary hypertension is a specific type of high blood pressure without a clear cause. Secondary hypertension, on the other hand, refers to high blood pressure that is caused by an underlying medical condition or factor, such as kidney disease, sleep apnea, or certain medications.