Orientifold
In theoretical physics, orientifold is a generalization of the notion of orbifold. The novelty is that the non-trivial element(s) of the orbifold group includes the reversal of the orientation of the string.
Orientifolding therefore produces unoriented strings - strings that carry no "arrow" and whose two opposite orientations are equivalent. Type I string theory is the simplest example of such a theory. It can be obtained by orientifolding type IIB string theory. The locus where the orientifold action reduces to the change of the string orientation is called the orientifold plane.
The orientifold plane in type I string theory is the spacetime-filling O9-plane. More generally, one can consider orientifold Op-planes where the dimension p is counted in analogy with D-branes.