There's no set definition for street level. It varies anywhere from "Someone with no powers, only skills and such, like Batman or any martial artist" to more broad definitions like "Very few powers (i.e. Cap, Daredevil, etc."
The furthest I've ever seen it go is around Spider-Man level, though most wouldn't define him as street level. There was a tourney a while back that I was in charge of, and people thought we were saying Carnage was street level...which we weren't, since we extended the limits beyond a normally accepted version of "street" to include more characters.
I realize that's not a concrete answer, but that's about the best you're going to get.
it's basically a low-level superhero, in terms of power, who generally operates on his/her own. x-men, avengers; they typically don't count. daredevil, as i see it, is the perfect example of a "street level" hero.
My definition of "street level" is no powers (Punisher, Captain America, Batman, Bullseye, etc) up to extremely low level superhumans eg, Dare Devil, Wolverine, Deadpool.... anyone up to around a ton or so in lifting ability not including energy using heroes such as Cyclops (whose destructive power gets him into a higher class IMO)