What makes a blog a blog?

I'm curious. What makes a blog a blog? Is it when you sign up through blogger? Or wordpress? Does that automatically make what you write into a blog? Or does it take some amount of creativity and a really cute background to make a blog a blog? Or maybe, you need a free seo tool to make your blog/website ranked number one on Google? Or maybe, it really just doesn't matter lol. I'm thinking it's the latter. I have a blog and it's a blog because well, I said so. And I write stuff. Lots of stuff. Lots of bloggy stuff. See, I have a blog. And that makes me happy. Yay. :)