In the comments of yesterday’s entry, the term “expanded universe” came up, as an alternative to “series”. Now, as much as I like the term “expanded universe”, and as much as my long-term plans would probably fit under that category… I’d imagine actually using that term in a query letter would be a red flag for agents and publishers.
“Expanded universe” means, to a degree, you’re thinking of a franchise. Which implies a property that has a TV, movie or video-game tie-in. It could also imply an author is a “big dog” at his or her publishers. David Weber at Baen is a prime example, as now there are other Baen authors writing in the Honorverse. Hell, it’s CALLED the Honorverse.
(Though, amusingly, the first Honor Harrington book has some significant differences from the books. It is the only one with an alien civilization– everything else is Humans Only (unless you count the treecats as intelligent aliens, which… well, I won’t get into that.)
“Expanded universe” also, at least to me, implies multiple authors. Which goes back to franchise.
What it really comes down to is worldbuilding, living, breathing worldbuilding that goes far beyond the borders of your first book.