The unit of resistance is a measure of the opposition to the flow of electric current. Abohm (5 letters) is a unit of electrical resistance equal to 10⁻¹⁸ ohms, Ohm (3 letters) is the SI unit of electrical resistance equal to one volt per ampere, Megohm (6 letters) is a unit of electrical resistance equal to one million ohms, and Rel (3 letters) is a unit of electrical resistance equal to one thousandth of an ohm.