May, as in "I may go" (9,4)
I believe the answer is:
auxiliary verb
(Other definitions for auxiliary verb that I've seen before include "To do, say" , "Part of speech that combines with another of the same kind" , "Predicative word defining the mood or tense of another" , "Be, do or have, for example, grammatically -- I buy Rex a rival (anag)" , "Part of speech such as "will" in "he will go"" .)