Subplot: miniature stories within the main story, that may include the protagonist or supporting characters, and that interconnect with the main plot, sometimes causally, sometimes thematically.
But wait, you've never heard of 'superplot'.
I propose the concept. Superplot:plot::plot:subplot.
After thinking about this for a while, I googled 'superplot'. There are couple of uses on the net with respect to fiction, but they're not talking about what I'm talking about. They're using 'superplot' to mean 'large story arc', a season-long or multi-season-long arc of a TV show, or the overall story arc of a trilogy of novels.
That's still 'plot'. And it's not what I'm talking about.
What am I talking about? I propose 'superplot' as a world-story. Not just static world-building -- planetary mechanics, climate, races, culture -- but a world that has its own ongoing, causally-related events, a world-sized story, immense in space and time. The plot is shaped by the superplot, but the plot influences the superplot little, if at all. Why? Because the plot, and its actors, are too small.
1. In the Lord of the Rings, the plot is Frodo and Sam taking the Ring to Mount Doom and destroying it. Subplots include the battle of Gondor, the romance of Eowyn and Faramir, the fate of Saruman, and many others. The superplot is the fall of Melkor. That was the event that started everything into motion, that precipitated the fall, and all subsequent actions of Sauron, that brought the Eldar and the Ishtari back to Middle Earth, and so on. The events set into motion by the fall of Melkor roll on for thousands of years, starting long before the start of LotR, and continuing long after. The events of LotR fixed Sauron's hash, but otherwise affect the events of the Fall very little. The Age of Men arrives, the other races fade away, the elves return to the True West, and so forth. I think that its superplot is one of the things that makes LotR seem so *rich* to me: I have the feeling of walking over almost limitless depths of history, that is still moving under my feet. Or: like being a little kid standing in a train station as the train roars by: from somewhere distant to somewhere equally distant, large, powerful, mysterious, and utterly beyond my ability to control or even affect.
The superplot of LotR is the plot of the Silmarillion, of course. The Silmarillion has no superplot per se.
2. In Lovecraft's "The Call of Cthulhu," the superplot is the history of the Great Old Ones, having ruled before and left traces, now dreaming and influencing susceptible humans, and fated to eventually emerge from their tombs and rule the world again. This is the background against which the characters in the story move, and which, in fact, is the prime mover behind the story events. Nothing the characters do, however, influences the superplot at all.
3. George Eliot's Middlemarch. The superplot is the events leading up to the Reform Bill of 1832.
4. Miyazaki's Porco Rosso. The superplot is the fallout of WWI, the gradual rise of fascism in Italy during the time of the Lost Generation, and its gradual but inexorable impingement on the hedonism of life around the Med.
I think superplots are found most often in novels. Many short stories, perhaps most, lack them. Even many novels lack them. For example, William Gibson's novels, though I admire them immensely, and although they have brilliant static worldbuilding, do not seem to have any identifiable world-size ongoing events larger than the characters and their story. Tim Powers' novels that I have read (not many of them) are the same: wonderful story-level worldbuilding, but no matrix of larger events in which the story is embedded. Pullman's "His Dark Materials" is the same: the characters are acting at the largest scale in that universe.
In some novels I distinctly feel the lack of a superplot, as though it is needed but not present. For example, Dodie Smith's much-loved I Capture the Castle is set in the 1930s. There's great cultural color and texture, but no mention of the momentous external events of the time: a world-wide depression, the rise of fascism, and the increasing threat of war. I liked this novel, but IMO, if the obvious superplot had been present, it would have added depth and poignancy to the story.
In summation: I am in favor of superplot. It's not always necessary, especially in short stories, but it can add wonderful depth, richness, and sense of wonder to novel-length fiction.