What is reactance?

Answer 1

The influence of a coil of wire upon an alternating current passing through it, tending to choke or diminish the current, or the similar influence of a condenser; inductive resistance. Reactance is measured in ohms. The reactance of a circuit is equal to the component of the impressed electro-motive force at right angles to the current divided by the current, that is, the component of the impedance due to the self-inductance or capacity of the circuit.

Answer 2

The electrical term reactance speaks to the "resistance" to AC in a circuit (though it isn't resistance) because of the presence of inductive and capacitive components in that circuit. Reactance can be either inductive or capacitive, and in each case we investigate, energy is taken from the circuit and then returned to the circuit, but at different times. Additionally, in the case of inductive reactance, the energy is stored in the form of a magnetic field. In capacitance reactance, the energy is stored in the form of electric charge.

When we consider the manner in which these two types of reactance operate, we might consider that they are actually complimentary. To cite a single example, when an inductor and a capacitor are in parallel in a circuit, they form a parallel tank circuit. In this circuit, the inductor and capacitor are taking energy and storing it, and then returning that energy back into the circuit at "opposite" times relative to each other. This allows the tank to "ring" or "oscillate" with a current circulating within the tank when it is at resonance. Links can be found below to begin investigating these phenomenon.

Comment

It might be better is reactance is equated to 'opposition' of a circuit, rather to its 'resistance'. The problem with the term 'resistance' is, that while (as in the above answer) it can be used in the sense of 'opposition', it has a very specific meaning in electrical engineering and, therefore, its use can be misinterpreted.