What is a subject matter expert?

A subject matter expert is someone who knows a lot about a thing. My methodology for finding them is to look for longer answers with a lot of upvotes. I'm a subject matter expert in music, along with related technologies, cultural history and underlying theories of emotion. I'm also a passionate amateur science geek and know more than the average person about evolutionary biology and assorted topics in physics. Also, by virtue of being married and having a kid, I am apparently some kind of expert on relationships, or at least the Quora community thinks I am.

Technically, a subject matter expert is someone who has much more then common knowledge on a subject.  On a  software team they are generally the person who knows what the customer does. You can tell a subject matter expert to some degree by reading a person's bio but that neither tells you everything nor is it necessarily accurate. In answers, a subject matter expert tends to give qualified advice because an expert answer to a question is usually neither a yes nor or no but a range of answers depending on circumstances.  On the other hand, just to be confusing, sometimes a subject matter expert will be able to boil all they know down to a pithy "rule of thumb" that covers the most general cases. Myself? Lets see, I've been a video game programmer for over 30 years, so thats an expertise.  I've stayed married over 25 years, the majority of which were happy,  so I'll claim some expertise there.  I'm an old fart, which means i have a moderate amount of experience with life, engineering careers, and so forth. I am not a true subject matter expert on religion and theology BUT I am an interested amateur and am married to a subject expert who backs me up so I'm probably almost as good as one there. I've had a medically controlled affective disorder (clinical depression) for most of my life.  So again. I'm not a subject expert but I do have some decent perspective on it.  I've been curious about the mains forever and have studied neuroscience and psychiatry on an interested amature level, which again makes me moire then a casual observer or patient but less then a full qualified researcher in the field. I have been a magician for about 40 years and I guess can be called somewhat of a subject expert the Oh and I've been playing pen and paper role-play games pretty much as long as they have existed and am definitely a subject matter expert there. Edit: Oh, and I AM NOT a subject expert on LGBTQ issues, being a straight white male who has not studied such academically. HOWEVER the common base of knowledge in this space is so woefully low in the general straight community, that compared to most of them, I'm pretty knowledgable.  This knowledge being second hand things learned from people who are.

Parts 1 and 2 have been covered off well by others here. Re: part 3 - what am I a subject matter expert in? Well, I'm not sure myself! I answer questions on London, retail working, survey queries, vegetarianism,  pets, all sorts. I don't think I'm an expert on anything as I don't have a depth of knowledge, but I have a wide breadth of knowledge, from life experiences, having a brain that retains useless information for a long time, and from general curiosity about the world. I get really enthused about subjects and read lots about then, learn facts and then get bored and move on. That information stays in my head until I go to a pub quiz or I see an appropriate question on Quora. So, in summary - I know something about a lot of things and nothing about a lot more things. I'm not a subject matter expert, unless the subject in general knowledge!

A subject matter expert has proven expertise on a subject. They are called upon in court rooms by lawyers to help determine the truthfulness of a specific issue on trial and being decided. For example-- A forensic expert witness has sufficient knowledge to testify in a legal case. They probably have 20+ years experience, maybe an advanced degree in their occupation, and perhaps they have a published book on their subject in which they are called upon to testify. They have provable knowledge as opposed to an undocumented opinion. DISCLAIMER--This answer does not come from an expert on any topic or subject. Just a senior citizen who has been paying attention and went to college for 17 years, and reads a lot. The reader is encouraged to cross-reference this answer to determine if I am even close to correct.

I am an SME in agile software development.