The Collaborative International Dictionary of English v.0.48:

modal auxiliary \mo"dal aux*il"iar*y\(Gram.), n.
   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.
   [PJC]

WordNet (r) 3.0 (2006):

modal auxiliary
    n 1: an auxiliary verb (such as `can' or `will') that is used to
         express modality [syn: modal auxiliary verb, modal
         auxiliary, modal verb, modal]