Shown in the Inbound links section on the SEO overview and on the Backlinks page, this is the number of different pages that this specific page links to.

If a page links to running event sites like toughmudder.com, toughmudder.com/obstacles, and runningforbeginners.com/couch-to-10k, that counts as 3 outbound links, even if the list links more than once to the same domain. It does not count duplicate links more than once.