Lateral Flexion – When referenced from anatomical position, lateral flexion is decreasing the angle at the joint, or movement out of anatomical position.  
(this motion is in the frontal plane)

Lateral Extension – When referenced from anatomical position, extension is increasing the angle at the joint, or movement back to anatomical position.
(this motion is also in the frontal plane)

See Video Demonstrations:

Lateral Flexion Lumbar Spine