Source : Webster's Revised Unabridged Dictionary (1913)

Hair grass \Hair" grass`\ (Bot.)
   A grass with very slender leaves or branches; as the
   {Agrostis scabra}, and several species of {Aira} or
   {Deschampsia}.

Source : WordNet®

hair grass
     n : fine-leaved aquatic spike rush; popular as aerator for
         aquariums [syn: {needle spike rush}, {needle rush}, {slender
         spike rush}, {Eleocharis acicularis}]