Variable Resistance

A creature that has variable resistance can activate it a specified number of times per encounter as a free action. When the creature activates variable resistance, it chooses a damage type: acid, cold, fire, lightning, or thunder. (The creature can’t choose a damage type to which it is vulnerable.) Until the end of the encounter, the creature gains a specified amount of resistance to that damage type. This resistance replaces any resistance the creature already had against that damage type.

If a creature can activate variable resistance more than once per encounter, the creature can resist only one damage type at a time using variable resistance. See also resist.

Published in Monster Manual 2, page(s) 219.