"In surface navigation, a cross sea is a sea state with two wave systems traveling at oblique angles. This may occur when water waves from one weather system continue despite a shift in wind. Waves generated by the new wind run at an angle to the old, creating a shifting, dangerous pattern. Until the older waves have dissipated, they create a sea hazard among the most perilous."
A cross swell is generated when the wave systems are longer period swell, rather than short period wind generated wavesAmazing. I didn't even know this could happen. You learn something every day.
Text and image from Wikipedia, via The Soul is Bone.