Let me give up the pedantry immediately: mechanically they're the same thing. A customer who already bought something buys more. That's an upsell and that's expansion and there's no daylight between them at the level of what happened.
The difference is entirely in how you got there, and that turns out to be the whole thing.
Why the word is poisoned
Two examples, and everybody has met both.
"You want fries with that" is an upsell and it's about as clean as commerce gets. You're already buying food, fries are food, the offer costs you nothing to decline and takes two seconds.
Then there's the electrician. You called about an outlet. While they're in the house they find a problem with your junction box, and the fix is $13,000. It isn't a code violation. It isn't a safety issue. Nothing about it stops your house working.
Here's what makes that example so useful: it isn't fraud. The thing they'd install is real, it functions, and it does what they say. Nobody is lying to you. It's just that nothing about it is connected to anything you needed.
And it works often enough that they keep doing it, because of the conditions rather than the customer. You're dealing with something you don't understand, that frightens you, explained by a licensed professional you already decided to trust an hour ago. Manipulation doesn't get you because you're stupid. It gets you because you're human.
That's the association the word carries. When somebody hears upsell, that's the memory it lands next to, either their own or the one where an older relative spent money they didn't have.
The version in your business
The B2B one is quieter and it's the same shape.
A customer forwards a sales email to whoever they actually talk to at your company. What is this. I'm still in onboarding. I've got three tickets open. The last thing on earth I want right now is to buy more from you.
Wrong thing, wrong person, wrong moment, and none of that is the worst part.
The worst part is what it proves. If you're asking me to buy more, you don't know what's happening in my account. You don't know I haven't gotten a result yet. You don't know I've been fighting an integration for a month. Every future message from you is now worth less, including the ones that were trying to help.
One badly-timed ask doesn't just fail. It costs you the channel.
What the distinction actually is
I can get an upsell a lot of ways.
I can strong-arm you. I can charm you. I can scare you. I can catch you at a moment when saying no is more expensive than saying yes. Every one of those produces revenue and none of them touched your progress, your readiness, or whether you got more value from us than you had last quarter.
That's why the word describes a method rather than an outcome, and why the method can be reached from any direction, including the ugly ones.
Expansion names the version you can only get one way: the relationship got bigger because the customer got somewhere, and the next thing genuinely fit where they got to.
It doesn't have to be complicated. They're getting results, this add-on now makes sense, and it might triple the account or it might be incremental. They're burning through their capacity and they've also reached the point where the next tier's other functionality finally matters. Neither of those required anybody to be talked into anything.
Which is also why a price increase is not expansion
Same test, and it fails immediately.
A price increase isn't tied to anybody getting more value. Nothing about the customer's situation changed. They didn't decide to grow their investment in you, they were informed that it grew.
At best it's neutral and tolerated. At worst it's the thing that finally gives somebody a reason to leave who was already looking for one. Nobody has ever felt closer to a vendor after a price increase.
The revenue is real and it lands in the same bucket on the same report. It just didn't come from the relationship expanding, and reporting it as though it did is how a company convinces itself the motion is working when nothing about the motion exists.
So it isn't lipstick
If expansion were just a nicer word for upselling I'd drop it, because the market says upsell and I'd rather be found than precious.
But the words point at different methods. One can be reached by pressure, timing, fear, or a well-constructed moment where declining is awkward. The other can only be reached by a customer arriving somewhere and being met there.
The second one is the only version that compounds, because it leaves the relationship in better shape than it found it, and the account can therefore do it again.
Lincoln Murphy formally named and popularized Customer Success starting in 2010 and has spent 15 years connecting it to expansion revenue and commercial outcomes. Read The Premise.