Actually there is a legal definition, a Public Benefit Corporation has statutes in its articles of incorporation which legally commit the company to pursue a set purpose which supersedes the fiduciary responsibility of the corporation to shareholders. This is important because it provides some degree of legal protection from activist shareholders suing the company for making spending or policy decisions which don't directly maximize shareholder value. The body of law around this issue is still relatively murky, but some defense is better than none at all.

For example, shareholders could attempt to sue BlueSky into increasing advertising placement or data sales functionality intothe core platform to increase company revenues, but if that is at odds with their stated public benefit purpose the legal team for BluSky would have grounds to attempt to dismiss the suit on the grounds that the shareholders purchase the shares under the explicit understanding that these functions would be subordinated to the public benefit goals of the platform.

Be strong now. Mastodon is a gGmbH. That's the German version and translates to public benefit limited liability corporation.