intermediate vector boson

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

• noun Any of three massive gauge bosons (the two charged W bosons and the neutral Z boson) that have one unit of spin and mediate weak interactions.

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

• noun physics Any of the three fundamental particles (the two W-bosons and the Z-boson) that transmit the weak nuclear force and have a mass about 100 times that of a proton.

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

• noun a gauge boson that mediates weak interactions between particles