As a mathematician, I tend to disagree with this common truism because it limits one's ability to think about transfinites and suggests there's a widely agreed upon technical definition of "number".
Ex-mathematician here, almost certainly in a different "circle," no there isn't. There are widely accepted standard definitions of things like integer, rational, real or complex number. But "number" is not really well defined. Frege's Foundations of Arithmetic contains a classical exploration of this exact question if you'd like some perspective.
How can you be an ex-mathematician? Maybe is a language barrier (English is not my main language), but I thought that was a designation based on your knowledge.
At least where I'm from, if someone were to say that they are a mathematician, they usually mean they are employed as a mathematician. So people will say ex-mathematician so the listener does not reach the wrong conclusion.
Thank you vm for the explanation, I was under the wrong idea that "mathematician" meant someone that knows mathematics, and just as being a hacker or an erudite, it couldn't be lost. Btw, I am a Financial Math PhD candidate, you saved me from potentially awkward conversations.
Don't worry. It wouldn't be a faux pas or anything. People often say "I studied math." (US) or "I read [past tense] maths." (UK) to mean they have mathematical knowledge.
PS: I've never been to the UK. I only put that bit in to have the brits explain it to you.