The 180 Degree Rule is a screen direction rule that camera operators typically follow whereby an imaginary line on one side of the axis of action is made (e.g., between two principal actors in a scene and the camera) that the camera must not cross; otherwise, there is a distressing visual discontinuity and disorientation.  This is similar to the axis of action (an imaginary line that separates the camera from the action before it) that should not be crossed.