git often uses the term “reference” in error messages

$ git switch asdf
fatal: invalid reference: asdf

$ git push
To github.com:jvns/int-exposed
 ! [rejected]    main -> main 
error: failed to push some refs to 'github.com:jvns/int-exposed'

“ref” and “reference” mean the same thing

Illustration of a tiny worried-looking stick person with a thought bubble reading “!”

“reference” often just means “branch”

in those two error messages, you can replace “reference” with “branch”

in my experience, it’s:
96% “branch”
3% “tag”
3% “HEAD”
0.01% something else

it’s an umbrella term

Illustration of git, represented by a box with a smiley face

git, thinking: “well, I COULD check if the thing we failed to push is a branch or tag or what, and customize the error message based on that….”
git, thinking: “seems complicated, let’s just print out “reference””

sad person: “why?”

reference: the definition

References are files: either .git/HEAD or files in .git/refs. There are 5 main types.

Here’s a list of every type of git reference that I have ever used:

• HEAD: .git/HEAD
• branches: .git/refs/heads/BRANCH
• tags: .git/refs/tags/TAG
• remote-tracking branches: .git/refs/remotes/REMOTE/BRANCH
• stash: .git/refs/stash

all of these files contain a commit ID, but the way that commit ID is used depends on what type of reference it is

(examples of more obscure references are .git/FETCH_HEAD and .git/refs/notes/... but I’ve never needed to think about those and your repository probably doesn’t even have notes)

git’s garbage collection starts with references

the algorithm is:

1. find all references, and every commit in every reference’s reflog
2. find every commit in the history of any of those commits
3. delete every commit that wasn’t found