supertall
n. An extremely tall building, particularly one with a height of at least 300 meters (984 feet).
Examples
2016
Work is moving forward on the SHoP Architects-designed supertall at 111 West 57th Street, YIMBY reports. Construction on the super slender tower is now about three stories above ground, and will rise to 1,428 feet when complete.
—Tanay Warerkar, “SHoP's Slender Midtown Supertall Finally Begins to Rise,” Curbed New York, June 06, 2016
2015
When it comes to supertalls, the main concern is wind.
—Clay Risen Posted, “Where Does The Glass For The New Supertall Skyscrapers Come From?,” Popular Science, August 03, 2015
2014
While Manhattan's new buildings have always tended towards tall, the current wave of projects pushing the city back to the top of the global totem pole are "supertalls": skyscrapers standing over 1,000 feet. This is a good thing.
—Nikolai Fedak, “How I Learned to Love Supertalls (And Why You Should Too),” Gizmodo, October 03, 2014
2003 (earliest)
Although I've had some great times in B-more, its skyline leaves a lot to be desired. It has decent density, but should have some supertalls.
—Big Rick McGee, “Baltimore, US” (reply), SkyscraperCity, June 10, 2003
Notes
The word supertall as an adjective has been around for a long time (since at least 1931), but its life as a noun has been much shorter, hence this entry.

When a building gets really tall (I'm looking at you Burj Khalifa), the prefix super- just doesn't cut the descriptive mustard, so once you get over 600 meters (1,968 feet), then the building becomes a megatall. Currently, the world has three megatalls (the Ping An Finance Center in China, at 599m, just misses the cutoff) and 127 supertalls (although some of those are under construction).
Supertalls and megatalls. Source: Wikipedia
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