modal auxiliary: Meaning and Definition of

mod'al auxil'iary

Pronunciation: [key]
  1. any of the group of English auxiliary verbs, including can, could, may, might, shall, should, will, would, and must, that are used with the base form of another verb to express distinctions of mood.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.