Am I a fluent English speaker?

I don't know if I'm fluent in English, but I can read, write, think and understand a lot of english conversations without any problems. Actually, I don't know what it means to be fluent in a language, but as I wrote, I can understand English and start conversations with English speakers as well.
I noticed that it's not a problem to switch from Italian to English and vice versa every time I want.
But the  problem is that I still make errors!
English speakers can understand me, but I feel myself a stupid by committing these errors.

What does it mean to be fluent in a language? Can I define myself a fluent english speaker though I make errors?
I'd glad to know your answers about this subject.

Bye!