What is ‘term_group’ for ‘order_by’ in get_terms()?

I was just reading the Codex regarding get_terms();. What exactly is the parameter 'term_group'?

You can use it in (example): 'order_by' for get_terms();.

Hint: I could be all terms that share the same names, but have different slugs.

The idea of term groups was to have a term with multiple aliases.

This feature doesn’t seem to be fully backed and is therefore practically never used.