Why Impedance is the total opposition a circuit presents to alternating current, combining resistance with the reactance of capacitors and inductors. Because it is a form of opposition to current flow, it is measured in the same unit as resistance, the ohm, and gets the same symbol (the Greek capital omega). In Ohm's law form, impedance Z equals voltage divided by current.
Watch out The volt is the unit of electrical potential and the ampere is the unit of current; those are the quantities you divide to get impedance, not impedance itself. The coulomb measures quantity of electric charge.
Anything that opposes current, resistance, reactance or impedance, is measured in ohms.