Cross polarization (sometimes written X-pol, in antenna slang) is the polarization orthogonal to the polarization
being discussed. For instance, if the fields from an antenna are meant to be horizontally polarized, the cross-polarization
in this case is vertical polarization. If the polarization is Right Hand Circularly Polarized (RHCP), the cross-polarization
is Left Hand Circularly Polarized (LHCP). See also Polarization.
This term arises because an antenna is never 100% polarized in a single mode (linear, circular, etc). Hence, two radiation patterns of an antenna are sometimes presented, the co-pol (or desired polarization component) radiation pattern and the cross-polarization radiation pattern.
The cross polarization may be specified for an antenna as a power level in negative dB, indicating how many decibels below the desired polarization's power level the x-pol power level is.
Antenna - Theory