n. A sport that involves walking or balancing on a slack nylon webbing suspended between two points.
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If you saw the Super Bowl halftime show, you probably wondered, "Who’s that guy in a toga bouncing crazily on a rope next to Madonna? And how’s he doing it?" The guy was Andy Lewis, a slacklining champion from California, and he did it after many, many years of practice. Slacklining is different from tightrope walking. Instead of a taught line, it’s performed on inch-thick nylon webbing that stretches and bounces.
—Marc Silver, “After the Super Bowl, Everyone’s Curious About Slacklining,” National Geographic News Watch, February 08, 2012
The distinction separates slacklining from seemingly similar ventures, such as tightrope walking, in which the rope is stretched tightly and remains static. A slackline moves, however, and when a beginner follows the impulse to concentrate on keeping her feet still, it moves even more, swinging from side to side as the feet clench it with increasing intensity.
—Diana Saverin, “Slackliners find balance one step at a time,” Pittsburgh Post-Gazette, December 11, 2011
1998 (earliest)
Potter, a 26-year-old "slack liner," was tethered to the rope by a leash attached to a harness that was wrapped around his waist and legs….

Slack lining, or loose rope walking, is part sport, part mind game, part spiritual quest. It requires physical agility, precision and balance to walk atop a line stretched over a precipice.
—Nora Zamichow, “A spiritual quest on a rope—at 1,200 feet,” Los Angeles Times, November 28, 1998
The suspended nylon webbing is called a slackline. If the slackline is suspended high over the ground, it's called highlining. Other variants include tricklining (performing tricks on the slackline), waterlining (slacklining over water), and urbanlining (slacklining in a park or other urban location). And, yes, the dude in the toga during the recent Super Bowl halftime show was slacklining.