VISUAL DESCENT POINT− A defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot.
AIM 1-1-17 (12) If a visual descent point (VDP) is published, it will not be included in the sequence of waypoints. Pilots are expected to use normal piloting techniques for beginning the visual descent, such as ATD.
AIM 5-4-23 (b) Visual Descent Point (VDP). A VDP will be published on most RNAV IAPs. VDPs apply only to aircraft utilizing LP or LNAV minima, not LPV or LNAV/VNAV minimums.
According to Wikipedia, an older version of the AIM stated that:
The concept of VDP was developed by the FAA to encourage pilots to decide to initiate a missed approach prior to reaching the MAP, in a situation where the runway or its environment is not visible at a normal descent angle. Conversely, if the runway is visible at the VDP, the pilot may continue descent, following a standard descent angle to the runway, while being assured terrain and obstacle clearance. The VDP is always located prior to reaching the MAP, and is a more useful checkpoint for making the decision whether to continue on the approach or to go around than the MAP itself.