The rear most roof support structure on an SUV, minivan or wagon. It corresponds with the C pillar behind the rear doors, the B pillar between the front and rear doors and the A pillar on either side of the windshield.
Overly large D pillars are often called out because they make it hard to see cars in your blind spot. Most modern vehicles now offer Blind Spot Detection systems that attempt to alleviate this problem. They use radar to scan the area on either side of a vehicle and alert the driver via audible or visual cues if there is another vehicle in their blind spot.