Source : Webster's Revised Unabridged Dictionary (1913)

Yellowtop \Yel"low*top`\, n. (Bot.)
   A kind of grass, perhaps a species of {Agrostis}.

Agrostis \A*gros"tis\, n. [L., fr. Gr. ?.]
   A genus of grasses, including species called in common
   language bent grass. Some of them, as redtop ({Agrostis
   vulgaris}), are valuable pasture grasses.

Source : WordNet®

Agrostis
     n : annual or perennial grasses cosmopolitan in northern
         hemisphere: bent grass (so named from `bent' meaning an
         area of unfenced grassland) [syn: {genus Agrostis}]