The abbrevation for "Unresolved Romantic Tension", a cousin to UST.  A facet of many tv shows, movies, and other media such as fanfiction.  Occurs frequently between two main characters who either don't realize, or won't admit to harboring infatuation for the other. 

1.)The difference between UST and URT is that the former refers to things potentially leading to expressions of love of a more adult nature, while URT can refer to all characters, including younger ones who haven't experienced sexual desires at their particular age. 

2.)  Many cartoon characters start feeling URT as they grow closer to puberty.