Excelsior is the name of the superhero support group that premiered in Runaways Issue #1, Series 2. Former teen superheroes, Turbo and the one good version of the Green Goblin, started the group to help other former heroic youth deal with the feelings of discontent and trauma of living the life of a superhero who also had to deal with puberty and gym class. This is, of course, slightly over the top and meant to be a satire on aspects of modern society and the comic book world.
As Julie "Lightspeed" Powers, once a member of the kid group Power Pack, stated in the first meeting,
When my siblings and I were fighting crime, I thought it was all just fun and games. I had no idea I was actually being robbed of a normal childhood. With the help of a lot of therapy, I'm trying to get some of the innocence back.
The former heroes at the first meeting were, besides the aforementioned Lightspeed, Turbo, and the Good Green Goblin, Generation X's and X-Men's Chamber, Riccochet (a mutant who was shortly featured in Spider-Man as part of team known as the Slingers), and Darkhawk, who's been featured in Avengers and New Warriors.
Soon after they formed, Excelsior was recruited by a mysterious benefactor who would donate millions to the organization if they'd help take the Runaways away from a life of teenage crime fighting.
They are now based out of LA fighting crime on the West Coast so that more novice heroes will not pop up to fill the role.