Well, that's what the folks at the Federal Emergency Management Agency calls it in their World Trade Center Building Performance Study (FEMA 403/May 2002). Here's a quote from the Executive Summary:

"The events following the attacks in New York City were amongst the worst building disasters in history and resulted in the largest loss of life from any single building collapse in the United States"

(Actually, two unevacuated buildings collapsed)

Back to my own comments:

Both towers absorbed the force of their respective collisions, or else they would have failed (collapsed) immediately. However, their otherwise flimsy and under-protected-from-fire structures were not able to withstand the fires (and high temperatures caused by paper not jetfuel), and melted, leading to the failure of the upper floors.

I use the term flimsy, others might say "lightweight" or "neither-over-nor-under-engineered"

The lower floors could not repel the forces exterted by this failure and themselves failed.

No one below the collision level(s) should have died on inside WTC 1 and 2 and those people above the collision level(s) not overcome by smoke should have had a heat-shielded egress down from the upper floors.

But that would have cost the building rentable floorspace and required PANYNJ to retrofit the fire-proofing.

The buildings failed in under two hours and they are the only protected steel frame buildings to have done so. By contrast the One Meridian Plaza building in Philadelphia burned uncontrolled for 11 hours (19 hours in all) in 1991 and is still standing.