There's not a lot of hidden meaning in the name.

It's called The Avengers because that's the name of the 1963 comic book on which it's based. Why was that called The Avengers? Probably because…Stan Lee and Jack Kirby thought it sounded cool. (They were right. It does.)

But things get a little more complicated when you check out the UK title of the film: The Avengers Assemble.

Our buddies across the pond already had something called The Avengers in its pop culture: a 1960s series that following the adventures of a slinky couple of superspies. It's reasonably well known here in the States, but it's absolutely huge in Great Britain. (Source)

It would be as if there were an international blockbuster called I Love Lucy or Happy Days—we'd basically have no choice but to tweak that name.