What is Reactance?

Reactance is one element of opposition to current flow in an AC Circuit. In respect to Capacitance and Inductance, it is one element of Impedance (with Resistance being the other). 

The Reactance component is an additional opposition to the movement of electric charge that arises from the changing magnetic and electric fields in circuits carrying alternating current (AC).  Inductive Reactance is associated with the magnetic field which surrounds a current carrying conductor, and it's a function of frequency of the alternating current and the inductance of the conductor (an inductor essentially opposes changes in current, making those changes lag behind those in the voltage). 

The other aspect, Capacitive Reactance is associated with the changing electric field between two conductors separated by an insulating media - in effect a capacitor. This capacitor opposes current flow by causing the voltage to lag behind the current in an AC circuit. The Capacitive Reactance is then a measure of this opposition and is inversely proportional to the Capacitance and the frequency of the AC. 

Total reactance is the difference between Inductive Reactance where the current lags the voltage, and Capacitive Reactance where the voltage lags the current. 

Note: As Reactance is a function of frequency, it is not present in direct current circuits.