Go back to the roots: experience. An expert is someone who has repeatedly solved the concrete problem you are encountering. If your toilet leaks, an experienced plumber is an expert. An expert has a track record and has had to face the consequences of their work. Failing is part of what makes an expert: any expert should have stories about how things went wrong.
I associate the word expert with ‘the problem’ because we know that expertise does not transfer well: a plumber does not necessarily make a good electrician. And within plumbing, there are problems that only some plumbers should solve. Furthermore, you cannot abstract a problem: you can study fluid mechanics all you want, but it won’t turn you into an expert plumber.
That’s one reason why employers ask for relevant experience: they seek expertise they can rely on. It is sometimes difficult to acquire expertise in an academic or bureaucratic setting because the problems are distant or abstract. Your experience may not translate well into practice. Sadly we live in a society where we often lose track of and undervalue genuine expertise… thus you may take software programming classes from people who never built software or civil engineering classes from people who never worked on infrastructure projects.