A term for a 'vector' that only has an orientation, not a direction?

Question

A 'vector' in $ℝ^2$ has a magnitude and a direction defined on [0, 2$\pi$). How would one describe a similar quantity, but with only an orientation defined on [0, $\pi$)? One example of this sort of axial data is astronomical observations of a polarization field, which typically are visualized by little line segments to indicate that they are indistinguishable under a 180 degree rotation. People sometimes refer to these as "vectors". "Vectors" is clearly incorrect, but what to call them?