"just a bunch of "science-y" fluff (Star Wars, Flash Gordon)."
Science would be using technologies for research and discovery, using scientific method.
Star wars has a bunch of technology, but isn't about science. No more then a laundromat. You may use some machines and chemicals to create a reaction giving whiter whites, but there's no research or discovery, and barly any scientific method, except the most basic instructions.
Star wars, "Press button, blow up planet."
I think space drama is a more modern and accurate term.
They say TV drama, not TV opera, although they are the same thing.
"A dramatic work in one or more acts, set to music for singing and instruments"
Opera definition.
Film and TV now have less singing, at least by the main cast, but it's often still there and music all the way, again, usually. They are dramatic works of multiple acts.
So most TV and film are operas.
Id say space opera is more a catch all for drama set in space, other planets, space ships, or perhaps also very futureistic earth.
Not as much fun, but going by the definition.
Science fiction is something else again, that some space opera may fit into, and vice versa.
By definition you'd expect some science going on, not just washing clothes. And it would have to have some fictitious element.
Often science fiction is just used for things that proceed the science. Like suddenly, there a spaceship, with not explanation of how it was created.
In this case BSG wouldn't be science fiction, but is a space opera?!?.
Star trek would be both, as they often explain the science and do experiments and describe how the ship and it's technologies were created.
Something like that.