Plain English Meaning
The box on an instrument approach chart that tells you how low you can descend and how much visibility you need before you must either see the runway or go around.
Full Definition
The portion of an instrument approach chart that lists the lowest altitudes and visibility values authorized for completing the approach. It is organized by approach category (straight-in, circling) and by aircraft category (A, B, C, D, E), and shows the Decision Altitude (DA), Decision Height (DH), or Minimum Descent Altitude (MDA), along with the required visibility for each.