surge protector

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A portable device containing one or more electrical outlets that protects equipment plugged into it from a surge in current.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An appliance designed to protect electrical devices from voltage spikes by limiting the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun electrical device inserted in a power line to protect equipment from sudden fluctuations in current