• (n.) A half bastion, or that part of a bastion consisting of one face and one flank.
Dem'i·bas'tion noun [ Confer French demi- bastion .] (Fort.) A half bastion, or that part of a bastion consisting of one face and one flank. 
demibastion A two-sided fortification, or half a bastion, that consists of a wall facing forward and a wall facing a flank.
a work consisting of half a bastion, and hence having one face and one flank.