From The Collaborative International Dictionary of English v.0.48 [gcide]:
Drawbridge \Draw"bridge`\, n.
A bridge of which either the whole or a part is made to be
raised up, let down, or drawn or turned aside, to admit or
hinder communication at pleasure, as before the gate of a
town or castle, or over a navigable river or canal.
[1913 Webster]
Note: The movable portion, or draw, is called, specifically,
a bascule, balance, or lifting bridge, a turning,
swivel, or swing bridge, or a rolling bridge, according
as it turns on a hinge vertically, or on a pivot
horizontally, or is pushed on rollers.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
drawbridge
n 1: a bridge that can be raised to block passage or to allow
boats or ships to pass beneath it [syn: drawbridge, {lift
bridge}]