Thats what one of my business cards says I am.
Basically it means I have a set of aviation skills, and am prepared to take other people's money to use them for their benefit.
In my case it generally means either running various certification projects for small or medium sized aircraft companies, or writing articles, manuals, etc. for people who need such things.
Why do people pay somebody like me for this instead of doing it themselves? Usually I have skills (such as how to write the CAA certification reports for a foreign aeroplane somebody wants to sell) that they very rarely need, so it's not worth keeping current within their staff.
There's no standard skill-set for a consultant, I've met people doing everything from aircraft certification, test-flying, procedure writing etc. to a very knowledgeable chap who does failure analysis of broken aeroplane parts, via a friend who designs avionics installations for one-off experiemental aeroplanes.
A firm of consultants is just a group of people, probably with complementary skills, who have banded together to do the same thing. Sometimes this can be a couple of people with disparate skillsets, sometimes a large enterprise filling an industrywide need and which has become a substantial company in it's own right.
I don't do this full time (but it does pay for my private flying and the odd holiday), but many others do. Pretty much everybody I know who became a consultant did so because they were working in an area where it became apparent that something needed doing, somebody was prepared to pay for it, and they could do the work better than anybody else available. Given that, although professional qualifications are relevant - the bottom line is your reputation and track record.
G
N.B. I've not personally ever joined
the British Association of Aviation Consultants , has anybody else, and was it worth it?