The Terminal High Altitude Area Defense (THAAD) system is a long-range, land-based theater defense weapon that acts as the upper tier of a basic 2-tiered defense against ballistic missiles. It’s designed to intercept missiles during late mid-course or final stage flight, flying at high altitudes within and even outside the atmosphere. This allows it to provide broad area coverage against threats to critical assets such as population centers and industrial resources as well as military forces, hence its previous “theater (of operations) high altitude area defense” designation.
This capability makes THAAD different from a Patriot PAC-3 or the future MEADS system, which are point defense options with limited range that are designed to hit a missile or warhead just before impact. The SM-3 Standard missile is a far better comparison, and land-based SM-3 programs will make it a direct THAAD competitor. So far, both programs remain underway.