The Collaborative International Dictionary of English v.0.48:

Ahem \A*hem"\, interj.
   An exclamation to call one's attention; hem.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ahem
    n 1: the utterance of a sound similar to clearing the throat;
         intended to get attention, express hesitancy, fill a pause,
         hide embarrassment, warn a friend, etc. [syn: hem,
         ahem]