From The Collaborative International Dictionary of English v.0.48 :
Sagebrush \Sage"brush`\, n.
A low irregular shrub ({Artemisia tridentata), of the order
Compositae, covering vast tracts of the dry alkaline
regions of the American plains; -- called also sagebush,
and wild sage.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
sagebrush
n 1: any of several North American composite subshrubs of the
genera Artemis or Seriphidium [syn: sagebrush, sage
brush]