Density Altitude
Also: DA
The altitude at which the standard atmosphere would have the same air density as your current conditions. It can differ widely from elevation.
Density altitude can be very different from elevation above sea level. A hot, low-pressure summer day at 1,500 feet might have a DA of 5,000 feet. The air is thinner than the geography suggests. A cold, high-pressure winter day at the same spot might be DA-500.
For long-range work, dialing the right DA into a ballistic app changes 1,000-yard come-ups by 0.2-0.5 mils. Carry a Kestrel or use an app with live weather to know your actual DA, not assumed numbers.