auxiliary verb

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

• noun A verb, such as have, can, or will, that accompanies the main verb in a clause and helps to make distinctions in mood, voice, aspect, and tense.

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

• noun grammar A verb that accompanies the main verb in a clause in order to make distinctions in tense, mood, voice or aspect.

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

• noun a verb that combines with another verb in a verb phrase to help form tense, mood, voice, or condition of the verb it combines with