So many good and thoughtful replies! Thank you.
It's really not such a straightforward question, huh? The responses have given me a lot to think about.
Generally, I don't like the question when asked. I have ideas how I think about the term 'professional', but I can never be sure what people mean when they ask.
As for the answers you all have given, is that generally how you respond to others when asked? I mean, I don't imagine most people who ask are looking for a dissertation or our life stories on what it means to be a "pro". What are your default answers when asked?
I don't remember what I say, but it's probably something along the lines of: 'I do some paid jobs, but mostly photography is my hobby.' That's probably as succinctly as I'm willing to put it, short of "No".
I was about to ask you why you asked the question and you just answered that. Generally, if people are asking the question of whether you are a pro, they are trying to make light conversation and the meaning behind it is a compliment, so just saying "no" in response would be rather curt. I think your answer (or slight variations of it) is perfect for the situation you described. If the person asking has a nice camera, you can add, "And, what about you?" More than that in a first round reply, however, would be too much. Your instinct that people are not looking for a dissertation is one to trust.