Bastion \Bas"tion\, n. [F. bastion (cf. It. bastione), fr. LL. bastire to build (cf. F. b?tir, It. bastire), perh. from the idea of support for a weight, and akin to Gr. ? 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.

Source: Webster's Revised Unabridged Dictionary (1913)