Copolarization and cross polarization refer specifically to a radio communication terminal that both transmits and receives signal. The terminal often has two separate antennae for those two jobs. The antennae are dipoles and so the electric field has an orientation relative to the antenna. Typically the antenna is a rod (or split rod) and the charge polarizes along the long axis of the rod, so that is the orientation of the electric field, the “polarization” of the radio wave. Other polarizations, such as right and left circular polarization are possible, but the principle remains the same. Copolarized and cross polarized refers to whether the two antenna have the same polarization or are oriented at 90 degrees to each other so they have orthogonal or “crossed” polarization. The advantage of crossed polarization is that the receive antenna is not sensitive to the transmitted polarization so you don’t blast your receiver with your own transmission and drown out the incoming signal. The disadvantage is that the distant terminal has to know and agree on what polarization to use because it has to do the opposite of what you choose. I work in optical communication, and we have the exact same issue.