Definition of "slacklining"

Highlining was a high-wire version of slacklining, an extreme cousin of tightrope walking in which no pole was used for balance and the rope was elastic, allowing for various tricks involving walking, sitting, lying down, flipping, even spinning hula hoops.

2008 March 14, Jeré Longman, “900 Feet Up With Nowhere to Go but Down”, in New York Times‎