Reactance is similar to resistance, except that it applies only to the AC characteristic of an electric current. Reactance varies, depending on the whether the considered device is inductive or capacitive. Simply put, an inductor's reactance increases with frequency, while a capacitor's reactance decreases with frequency.

Here's the formula for an inductor's reactance:

Xi = 2pifL

Xi is your reactance, in Ohms.
f is your frequency, in Hertz.
L is your device's inductance, in Henries.

Here's the formula for a capacitor's reactance:

Xc = 1/(2pifC)

Xi is your reactance, in Ohms.
f is, again, your frequency in Hertz.
C is your device's capacitance, in Farads.