The Collaborative International Dictionary of English v.0.48:

Bastion \Bas"tion\ (b[a^]s"ch[u^]n; 106), n. [F. bastion (cf.
   It. bastione), fr. LL. bastire to build (cf. F. b[^a]tir, It.
   bastire), perh. from the idea of support for a weight, and
   akin to Gr. basta`zein to lift, carry, and to E. baston,
   baton.] (Fort.)
   A work projecting outward from the main inclosure of a
   fortification, consisting of two faces and two flanks, and so
   constructed that it is able to defend by a flanking fire the
   adjacent curtain, or wall which extends from one bastion to
   another. Two adjacent bastions are connected by the curtain,
   which joins the flank of one with the adjacent flank of the
   other. The distance between the flanks of a bastion is called
   the gorge. A lunette is a detached bastion. See Ravelin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bastion
    n 1: a group that defends a principle; "a bastion against
         corruption"; "the last bastion of communism"
    2: a stronghold into which people could go for shelter during a
       battle [syn: bastion, citadel]
    3: projecting part of a rampart or other fortification

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "bastion":
   abatis, acropolis, advanced work, balistraria, bank, banquette,
   barbed-wire entanglement, barbican, barricade, barrier, bartizan,
   battlement, beachhead, blockhouse, breastwork, bridgehead, bulwark,
   bunker, casemate, castle, cheval-de-frise, circumvallation,
   citadel, contravallation, counterscarp, curtain, demibastion, dike,
   donjon, drawbridge, earthwork, enclosure, entanglement, escarp,
   escarpment, fasthold, fastness, fence, fieldwork, fort, fortalice,
   fortification, fortress, garrison, garrison house, glacis, hold,
   keep, loophole, lunette, machicolation, mantelet, martello,
   martello tower, merlon, mote, motte, mound, outwork, palisade,
   parados, parapet, peel, peel tower, pillbox, portcullis, post,
   postern gate, rampart, rath, ravelin, redan, redoubt, safehold,
   sally port, scarp, sconce, stockade, strong point, stronghold,
   tenaille, tower, tower of strength, vallation, vallum, ward,
   work