A sequel

[Stina]

I think of this as being the second book of a duology, as opposed to the second book of a longer series. But that is not a hill I am willing to die on. And then you run into situations where declared duologies then get prequels or graphic novel continuations or simply keep on going in the manner of Douglas Adams's "increasingly inaccurately named Hitchhiker's Trilogy."

[Mary]

Wouldn't a third, fourth, etc. book of a series also be a sequel?

[Stina]

Mary wrote: "Wouldn't a third, fourth, etc. book of a series also be a sequel?"

Depends on whose definition of "sequel" you are using. Some sources are very specific that it is the second part of the story. Others, not so much.