What is a superhero?

There are many characters in pop culture that seem to hit all the checkmarks of being a superhero, but are not generally considered as such.

Luke Skywalker, Neo, Lelouch, TMNT, Buffy, Winchesters, etc.

Conversely, many characters are often lumped in with superheroes, despite seeming out of place among them.

Punisher, Constantine, Hellboy, Swamp Thing, Spawn, etc.

So what exactly makes a character a superhero? Is it powers, secret identity, fighting evil, saving the world... Or is it just being from Marvel / DC?

The name is self explanatory. A super hero

Super would imply superpowers

Hero is a matter of perspective

I think what makes a superhero a superhero is sort of that "cheese" factor, if you know what I mean. Costume/design, the overall world they live in. Maybe a rogue's gallery. The overall thing with saving the day.

a superpowered individual who commits heroic acts for the good of all, and has ties to comic book tropes. i.e. rouge gallery, team, costume. ect.

Beating street level thugs.

A hero, contrary to what people are saying… they DO NOT need superpowers. But they do need to be “super” in some way. Any skill or equipment that puts them beyond what the average person can do counts in this regard. For example Batman doesn’t have powers… but he put forth the time, effort, and energy to become something the average person cannot be. Hawkeye and green arrow have no powers either but can be considered superheroes because of their ability to shoot a bow. It comes down to being a person who saves lives but has some “super” factor that puts them above the average person. Does not have to have actual powers. I would say all those people you mentioned are indeed superheroes, but most people usually regard characters that have started in comics as superheroes