The Collaborative International Dictionary of English v.0.48:
Redoubt \Re*doubt"\ (r?*dout"), n. [F. redoute, fem., It.
ridotto, LL. reductus, literally, a retreat, from L. reductus
drawn back, retired, p. p. of reducere to lead or draw back;
cf. F. r['e]duit, also fr. LL. reductus. See Reduce, and
cf. Reduct, R['e]duit, Ridotto.] (Fort.)
(a) A small, and usually a roughly constructed, fort or
outwork of varying shape, commonly erected for a
temporary purpose, and without flanking defenses, -- used
esp. in fortifying tops of hills and passes, and
positions in hostile territory.
(b) In permanent works, an outwork placed within another
outwork. See F and i in Illust. of Ravelin. [Written