It defines when an engineer opens a pull request and then approves it him/herself. Heading unreviewed to production!
HOW TO RECOGNIZE IT
Self-merging is easy to see because the submitter and the reviewer are the same people.
WHAT TO DO
Many organizations prevent self-merging PRs by configuring their build systems to reject them.
Reviewing these PRs on a case-by-case basis, even AFTER being merged, will prevent bugs to get buried.
If engineers are in the habit of self-merging without review, talk with them to ensure that they understand the ‘why’ behind getting the review process or at least clear on expectations.
If they’re more senior, encourage them to follow the best practice of getting code thoroughly reviewed by others, so other engineers will model that behavior.