The unit of electrical resistance is a measure of the opposition to the flow of electric current. The two most common units of electrical resistance are the ohm (Ω) and the megohm (MΩ). The ohm is the SI derived unit of electrical resistance, equal to the resistance between two points on a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere. The megohm is equal to one million ohms.