I've come across answers on Stack Overflow that I've found particularly intriguing, but half way through reading I've had to break off and perform a Google search to find out what a particular term or phrase means. This often interrupts the flow of reading the answer, or sometimes I'll get distracted by the term and forget about the answer, only returning to it hours later when I'm cleaning up my unused tabs in the browser.
Reading through an answer I posted today, I realized that some of my own answers might contain terms that could be confusing to someone less familiar in the subject than I am. I think it would make Stack Overflow an even richer source of information if we could define these terms without cluttering up the post body with irrelevant material.
I can see at least several terms that might be confusing to some in this very post - what's Markdown?
<dfn> is the most semantically appropriate HTML element to use for this sort of thing.