modal auxiliary

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

• noun An auxiliary verb characteristically used with other verbs to express mood, aspect, or tense. In English, the most common modal auxiliaries are can, could, may, might, must, ought, shall, should, will, and would.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Gram.) Any one of the auxiliary verbs of English, such as can, may, will, shall, must, might, could, would, or should, which are used together with the infinitive form of another verb to express distinctions of mood{2}, such as uncertainty, possibility, command, emphasis, and obligation.

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

• noun grammar a verb used to express the mood (or tense) of another verb

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

• noun an auxiliary verb (such as `can' or `will') that is used to express modality