The older, still in use today meaning is what happened when Oracle bought MySQL and ruined it. People forked it and now we have MariaDB. Basically, it means a fork in the code base and now there are two separate projects.
Until Github came along, "Creating a copy of software for your own personal use" had never been widely-accepted definition of the word "fork" in the context of software development. Forking a project has always involved independent publication and maintenance of said project.
By "ruined" you must mean "solved decades-long issues and turned it into a real database", or we're talking about two different projects.
https://dev.mysql.com/blog-archive/the-complete-list-of-new-...
https://dev.mysql.com/doc/refman/8.4/en/mysql-nutshell.html
Do note that the largest users (like GitHub and Facebook) still use the original MySQL, and AFAIK have no plans to abandon it.
When you fork a project on GitHub, that literally creates a parallel working history, and publishes it under your username. That's what GitHub means by "fork".
So what's the other definition?
This sort of argument change how people understand words, and it also doesn't change how lawyers interpret laws nearly as often as people think. It's still fun, though!