noun.
One that upholds or defends something, as against neglect or unpopularity: synonym: bulwark.
The Century Dictionary and Cyclopedia
noun.
In fortification, a mass of earth, faced with sods, brick, or stones, standing out from a rampart, of which it is a principal part.
the GNU version of the Collaborative International Dictionary of English
noun.
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.