Since it’s not an official term anywhere, it’s never had any kind of official control of the definition. So people probably don’t all agree. And it may be evolving over time; language does that, especially English.
The Wikipedia article (at least as of today) seems to capture this idea, and has more details about the history and evolution.
It was a solidly pejorative term originally, when Wilson Tucker invented it. However, by the time I encountered it, it was at worst ambiguous.
As many of you probably know, I’m a big Doc Smith fan. He’s often considered the defining example of space opera, in the good senses. So I’m not willing to go with a solidly negative definition for the term.
I’ve found a lot of people using the term to just mean “science fiction action-adventure”, and I don’t find that a satisfactory definition. It’s too big a field, and there’s already a good term for it, so I don’t want to let them have mine.
To me, space opera has several characteristics:
- Very large scope. Size is measured from the protagonist’s point of view, so if they’re restricted to one solar system, saving that whole solar system counts; on the other hand, Doc Smith has his Lensmen fighting an extra-universal threat, and operating across two galaxies. The scope probably grows a lot in the course of the story.
- Science Fiction. And space travel.
- Discovery-driven. The action is required or made possible or both by new discoveries, either scientific or exploratory. New fields of science or the universe (or both) are constantly being opened up.
- Protagonist is dominant player in the discovery.
I don’t know if it’s a defining characteristic, or a common outcome of the other defining characteristics; but space opera tends towards moral clarity (or being simplistic; your choice).
So. Doc Smith is clearly space opera (Skylark and Lensman series in particular). George O. Smith’s Venus Equilateral is clearly space opera. But what about more modern works?
Well, Vernor Vinge’s A Fire Upon the Deep seems to qualify. It’s not quite focused on one protagonist, but the main characters play a really big role in saving everything. Lois McMaster Bujold’s Vorkosigan stories don’t, though; they’re not discovery-driven, and they take place in something of a backwater of even the human universe, yet with knowledge that the rest exists. (I love them to death anyway; my taste is not limited to space opera.) David Weber’s Honor Harrington series is borderline; the action takes place mostly in a small peripheral star kingdom, with the Solarian League visible but not involved in the background. Also, Honor isn’t the driver of much of the discovery, though she’s certainly terribly important in the action overall. They also tend a bit more than I prefer towards simplistic black-and-white views of events. Mike Shepherd’s Kris Longknife books fall in almost exactly the same place as Weber, I think; borderline.