When the imaginary Axis established by the 180˚Rule is crossed by the camera, this is called "crossing the line." For example:

Person A is on the left in the picture and Person B on the right.

Person A looks from left to right, Person B looks right to left.

If, after cutting to a close up of person A, person B appears stationary, person A is still looking from left to right. If in a close-up shot of person A suddenly looked right to left, the camera would have crossed the axis, a "corssing the line". The effect would be as if a person were looking in the opposite direction.

Crossing the 180 degree axis, resulting in a jump cut, can be disorienting to the viewer, this is sometimes an intended effect.

The new shot, from the opposite side, is also known as a reverse angle.