Professional photographer is someone that has a career as a photographer. You pay taxes on the money you make from photography, and is supports you then you are a professional.

You've been taking pictures for 5, 10, 15 years and have all the equipment know how, you are an enthusiast.

If you've done something in the past that you got paid for you don't automatically become a professional. A professional title is ongoing. My dad worked for the st pete times as a professional photographer for 5 years back when I was first born. He takes great pictures still but I don't consider him a professional photographer.