I get Stack Overflow (which I'm guessing was the first site?) and Super User, Code Review, and the other sibling site names make sense to me. But I've never heard of a stack exchange before.
Is a stack exchange a real thing, or is this site's name a combination of Stack Overflow and an exchange of ideas?
…or is this site's name a combination of Stack Overflow and an exchange of ideas?
I can't say with absolute certainty, but I'm fairly sure this is the correct explanation. While one might speculate there was some influence by the proximity of the FogCreek offices to the New York Stock Exchange, "StackExchange"* was originally intended to be sold as a product – a customized version of Stack Overflow for your own topic(s).
Products have marketing, and the promotional blurb on the FogCreek site at the time refers to the offering as the "StackExchange** Stack Overflow Knowledge Exchange platform", suggesting that the name is very likely a portmanteau of "Stack Overflow" and "Knowledge Exchange".
*The space was added sometime later, as it's consistently absent from the early material
**Joel points out in the comments this was actually a typo
The first public mention that I know of of the name "Stack Exchange" was in Stack Overflow Podcast #58 (streaming audio) (released June 17, 2009). Joel Spolsky and Jeff Atwood discussed naming from 00:04:40 to 00:08:00, and discussion of the name "Stack Exchange" in particular is mostly at the beginning of that segment.
Spolsky noted that Jason Calacanis had told them that the product they were offering could be referred to with the unfamiliar-to-Americans term "knowledge exchange." He had introduced them to the term in Stack Overflow Podcast #56 (streaming audio - 00:08:05 - 00:09:05), noting that it is used in Korea to describe websites that people use there to get information more than they use search.
So this name refers to that generic descriptor "knowledge exchange" as well as to the name of the original site, Stack Overflow, that the Stack Exchange product was based on.
