relativistic mass

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

• noun In special relativity and general relativity, the observed mass of an object moving with respect to the observer. The relativistic mass is a function of the rest mass and the velocity of the object.

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

• noun physics The increased mass of an object moving at relativistic speed relative to an observer.

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

• noun (physics) the mass of a body in motion relative to the observer: it is equal to the rest mass multiplied by a factor that is greater than 1 and that increases as the magnitude of the velocity increases