Expansions, to me, expand the base game or game up to the point of the expansion releasing. Except in the case of WoW, expansions tend to create a better, more robust game experience over time. With WoW, and its many expansions, some of them have been utter crap and others have really added to the game.
DLC's could be anything from paying $5 for a knife skin in your favorite fps, to an actual expansion marketed as "DLC" because that's the buzz word in the industry. Very few companies separate the two anymore, but to me DLC is a little more inclusive than the term "Expansion pack".
To have an expansion, you need to, as I said, expand the game. You need to make it better in some way, add new gameplay, races, characters, quests, storylines, something.
DLC has no such stipulation and again, as I said, could mean anything you download in addition to a game. You could pay $10 for a digital strategy guide that you don't need because Google is free, but hey... Us gamers are not the most rational and tend to spend money on crap we shouldn't.
Gamers are also the reason companies sell lootboxes, which technically to me are a form of DLC, and loot packs, which are also a form of DLC. You're spending in most cases very real money for a very small chance at a meaningless item or buff. We the gamers are directly responsible for that being a big thing. Ubisoft isn't wrong, the lootboxes and currency packs wouldn't exist if gamers didn't buy them.
I am ashamed of my hobby but it is my hobby. I am ashamed of my fellow gamers but they are my fellow gamers. And I am ashamed the industry keeps creeping towards attempted exponential profits and not towards making fun, quality games.
This whole discussion leads me to believe the AAA houses will all move permanently to the mobile sphere, and that the pc platform will be left to the indie and medium devs, and Microsoft because they always program for Windows, despite not always programming for linux or mac.
The evolution of expansions to DLC has become the evolution of DLC to lootboxes, which is becoming the evolution of lootboxes to more predatory, but perceptually less predatory, microtransactions and a shift to mobile to make more money.
It's all evolution and until gamers start voting with their wallets, it'll keep getting worse. This is just my opinion, sorry for the wall of text.