What's the difference between high blood pressure and hypertension?

Almost same: High BP is a single reading that can go up temporarily from exercise, stress, pain or illness. Hypertension is a treatable diagnosis of repeated high BP readings > 140/90 without acute stress. Other than that they are often used to mean the same thing.

Measurement : High blood pressure is when blood pressure is measured to be high, which may occur in normal individuals too. Hypertension is a medical condition with consistently high blood pressure readings, which needs to be treated.