Q. What is it?
A. It refers to not-really-public information that is legal to trade on. Examples: research reports that combine website cookie data and credit card info purchased from credit card companies.

Q. Is it legal?
A. Yes. “Insider information” is illegal, but not all non-public information is illegal to trade on. To be illegal, somebody somewhere has to be violating a duty of confidentiality. If you just collect information that’s “out there” in a way nobody else has, no problem

Q. What other sources?
A. A hot one: social media trends; FB, Twitter, Quora, etc. A funny one: satellite images used to count the cars in the Wall Mart parking lot

Q. What’s the SEC doing?
A. Can’t do much unless it requires companies to continuously report, which very eventually may well happen

Q. Popular on Wall Street?
A. Very – several banks have “evidence based trading” units now, and very popular with hedge funds – they’re always on the prowl for quasi-public info that will give them an edge.