From WordNet (r) 3.0 (2006) [wn]:
crossbow
n 1: a bow fixed transversely on a wooden stock grooved to
direct the arrow (quarrel)
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Crossbow \Cross"bow`\ (kr[o^]s"b[=o]`), n. (Archery)
A weapon, used in discharging arrows or bolts, formed by
placing a bow crosswise on a stock.
[1913 Webster]