If you answer more than 5 questions in a category and have the most points in that category.

see: http://www.answerbag.com/q_view/89668

basically, you need to answer at least five questions in the subject, and you must have at least 25 points earned on one of the answers

so it's not just points--you could have 1000 points in a category, but if they only come from one answer, if you don't answer other questions, you don't qualify

if there is no expert in the category already, and you answer five questions and get 25 points on an answer, then you're the expert. if there is already an expert, then you become the expert if and when you pass them in points earned in the category. there is one expert per category, but, while there may be one for a really large category, there are also ones (or there can be, someone may just not have qualified yet) for every single subcat

I think it is the person who answers the most questions on a particular category. It may be linked to how many points you get, but I am not sure about that.

There are no experts only astute people. Remember:- Expert? An Ex is a 'has been' AND a Spurt ( Spert) is a drip under pressure.