yep.
here's a definition of sport according to dictionary.com
–noun
1.an athletic activity requiring skill or physical prowess and often of a competitive nature, as racing, baseball, tennis, golf, bowling, wrestling, boxing, hunting, fishing, etc.
2.a particular form of this, esp. in the out of doors.
3.diversion; recreation; pleasant pastime.
Slacklining takes a good amount of skill and physical prowess, has a world championship every year, is practiced primarily outside, and many people do it as a form of recreation.
So yes, I would definitely say that slacklining is a sport.
the insert link button won't work, but here's the URL
http://dictionary.reference.com/browse/sport
