Plain English Meaning
The point during a precision approach where you must decide: if you can see the runway, you keep going and land; if you can't, you go around. It's a height above the runway, not above sea level.
Full Definition
On a precision instrument approach, the specified height above the runway touchdown zone at which the pilot must decide whether to continue the approach to landing or execute a missed approach. At Decision Height, the required visual references for the runway must be in sight; if they are not, the pilot must go around. Decision Height is referenced to the height above touchdown (HAT) and is published on the approach chart for procedures such as .