Here's my view:
Expansions and DLC are two different concepts but with a lot of overlapping ground between them.
An expansion:
- Typically substantial
- May or may not be a standalone product
- Does not always, but usually does, require the base product to run.
- Can be downloaded or bought physically (while that's still an option)
DLC:
- Could be anything between substantial and "just a palette swap"
- Can be entirely new content, or may be bundled with the base product and requiring activation (looking at you PDX)
- Always tied to the base product
- Is generally always downloaded, unless you've bought a physical GOTY or something