UEENEEG102A - AC circuits

a property of a circuit component, instrument or some other device which encapsulates the relationship between the potential difference (PD) across the device and the current through it. For steady (constant) PD and current, the impedance is equivalent to the resistance, the quotient of PD divided by current, but if the PD and current vary with time, you have to take into account the fact that there may be some delay between cause and effect - potential differences and currents may not change in unison. To specify impedance in such cases you have to know, as well as resistance, a second property called reactance which depends on the relationship among PD, current and the time scale of the changes. The two properties, resistance and reactance, together constitute impedance which cannot be described by a single numerical value. In DC circuits - with steady currents - the reactance of all components can be ignored, so in those cases impedance means the same as resistance. Some folks, such as loudspeaker salespersons, say impedance when they mean resistance.

Source: http://www.physics.usyd.edu.au/teach_res/db/elgloss.htm

Circuit Impedance is a combination of Resistance and Reactance in an AC circuit. 

All circuits both AC and DC (exc superconductive) contain Resistance (ohms). Only AC circuits contain reactance(ohms). 

Therefore when dealing with AC circuits to find the full opposition to power we must know the reactance(X) and the resistance(R), the correct mathematical combination gives us the circuit impedance(Z). 

Opposition to current flow caused by a coil when an AC supply is used.

The INSTANTANEOUS value of an alternating voltage or current is the value of voltage or current at one particular instant.