The term ‘series’ I suppose is mostly self explanatory, and family sagas are the first novels that come to mind, at least for me. However, if the plot dynamics are dissimilar and only one character is from the preceding novel, does this fit within the term ‘series’. Lee Child’s Reacher is an example. But is Nelson DeMille with his returning John and Kate Corey as FBI and NYPD agents (The Panther is his latest)? And if the novels have stand-alone storylines–again Lee Child’s are good examples, are they really ‘series’ novels?
I never thought to define the term series before. What’s interesting about this question is how it got me thinking about all of the different types of series there really are.
As you mentioned, there’s the family saga type series that might have a continuing plot line of sorts from book to book, often following generations of families. I haven’t read a book like this in eons so this one would never have even come to mind.
Mystery and suspense series usually have a protagonist (and her posse) as continuing characters in a series of stand alone novels. Usually the books completely stand alone, but there are often some plot lines or aspects of an earlier plot that will carry through. For example, each book might stand alone, but there might be a romance that continues throughout the series or an injury the protagonist receives in one book might plague him in later books.
Romance series tend to be set in a similar location, but tend to follow a new couple in each book. Occasionally, characters from other books will make appearances, but not always. Of course, there have been romance series that follow one couple and their adventures through the course of a number of books. There are no rules.
There’s a real definition of what makes a series, but I think readers usually attach themselves to something or someone and want to keep coming back to read more in that world. Whatever that world is makes a series.