Datum is Latin and means “something given”; the datum line is a marked point or base of reference for measurement.
When it comes to MOT testing, the datum line is referenced in conjunction with headlamp testing equipment. New MOT Centres must have DVSA approved headlamp testing equipment. There must be a datum line (or lines) clearly marked as per the equipment installation manual.
The MOT Guide, Section D makes reference to the need for datum lines. Information will also be found in the instruction manual of your headlamp aim equipment.