Density Altitude.....................basically it's a calculated altitude that factors in all of the necessary elements.
Example

My track is at 2200 feet elevation...........the DA the last time I ran put it at 5600 feet elevation(equivalent). So it was like I was actually running at 5600 feet which relates to much slower times than those who are lucky enough to be at/near sea level where you can actually achieve a NEGATIVE DA at times.