This command fetches all tags:
git fetch origin --tags
This command fetches a specific tag:
git fetch orig
I read all the answers, but there is not yet mentioned one syntactic sugar. If you need to fetch only one tag as a tag (to checkout later) you can write, for instance for refs/tags/2.3.18:
git fetch origin tag 2.3.18 --no-tags
This is shortcut for already mentioned:
git fetch origin refs/tags/2.3.18:refs/tags/2.3.18 --no-tags
Of course, you may not use --no-tags if you need other tags (according to the default behavior) or if you already explicitly set --no-tag in the clone command or in tagOpt config option(man git-clone).
git fetch origin tag 2.3.18