It's called es-zett (S Z), and I believe it originally was a joined long s and a z, like ſ𝔷. (There doesn't appear to be a fraktur long s in unicode, but I include the fraktur z to show where the shape comes from.) I believe these days it is typically written as "ss" when the ß character is not available.
Edit: The name points to sz, but it's possible that it replaced both ss and sz. I'm not an expert.
Edit: The name points to sz, but it's possible that it replaced both ss and sz. I'm not an expert.