A sofa has two cushions (also known as a love seat) while a couch has three cushions.
I appear to be in the minority however.
That’s pretty much the opposite of what I said.
Don’t even get me started on ottoman.
I agree with you and assumed that is what everyone thought until I read this thread. A sofa to me doesn’t have a strict definition but it tends to be a smaller sized couch like a love seat. Couches are the big ones in my mind.
For me it’s always been that sofa, couch, davenport and chesterfield are synonymous terms for the largest (usually three-seater) bit of living room/lounge room/parlor furniture.
A love seat looks like a sofa but has two seats – it’s for lovers to sit alone and cuddle, hence the name, and lack of seat for a third wheel.
A settee, on the other hand, contrary to my Commonwealth friends, is specifically something set off from the ground with a decorative back or arm pieces and always with the higher, typically spindled and exposed legs and almost always just two seats, often resembling a chair that mutated much more than it resembles a sofa. This may be called a settee to some but to me, the skirting at the bottom makes it 100% love seat/sofa.
How about divan? Synonymous to the other terms, or different?
I’ve never figured out how any number of those things would constitute an empire.
I knew a girl who would perch a certain way on the ottoman. And she was just getting started.
I use the two interchangeably, but to me a sofa is usually a bigger version of a couch.
I always say sofa. Couch is the same thing.
I was going to ask the same thing. I think all three mean the same thing. And I agree with the poster above that a setee (in the US at least) is more like a big-ass chair than a sofa/couch/divan/etc.
I always think of a sofa as an uncomfortable couch. Davenports are really uncomfortable couches and probably have furniture covers on them to boot.
That’s a specific type of couch. Like calling all cars hatchbacks no matter what.
Edit:
In other news, I was born in Chesterfield, UK. You may not, however, sit on me.
In the UK a divan is a type of bed with a solid base, often with drawers included in the base.
Perhaps in the UK. In Ontario there’s no distinction in general. Well, maybe some distinction in the same way other Americans are trying to distinguish between sofas and couches 
And please don’t sit on me when visiting one of my favorite restaurants in Chesterfield, Michigan!
Divans: sofas that might have arms but definitely don’t have backs so they look like (but aren’t) beds to an extent.
These are labeled as divans, but they aren’t. Because they have an arm and/or an incline at one end they’re chaises.
“Window treatments”?? Is that like windex? 
nm
Put me down for the same logic and word. If it’s bigger than a chair and will seat at least two it’s a couch or a sofa, even it there may be a fancier name for the specific design. Other names I have heard for such a piece of furniture include settee, divan, chesterfield, and others I can’t remember at the moment.
Sofa and couch are interchangeable to me.
None of the above. I say ‘lounge’.