Note what that definition includes and what it does not. It is sustained, it concerns perception of reality rather than the facts of one dispute, and its effect is on confidence rather than on the argument.
The online version
Online it usually shows up in a narrower form than the clinical one, and the distinction matters.
A message is edited and the earlier version denied. A statement made in front of dozens of people is described as something you invented. Your objection is recharacterised as an overreaction, then your reaction to that is offered as proof you were overreacting all along. Each step is deniable, and the record — if there still is one — is a screenshot you now have to produce like evidence.
The effect that matters is not losing the argument. It is the half-second of genuine uncertainty about whether you remember your own week correctly.
Recognising it
- Flat denial of things that were said in public, where a record exists
- Your reaction becoming the subject, repeatedly, in place of the thing you reacted to
- Edits, deletions, and then confident accounts of what was "actually" written
- A pattern across time rather than a single contested exchange
What to do
Keep the record, quietly. Screenshots with timestamps, kept somewhere else. Not to win an argument — to settle the question for yourself, which is the part actually under attack.
Do not litigate your own memory in public. It is the argument you cannot win and the one they benefit from, because every round of it performs uncertainty in front of an audience.
Check with someone who was there. One outside account of the same exchange does more than any amount of re-reading.
If this is happening across your life rather than in one thread, and by someone with ongoing access to you, that is a different situation from an online argument — the support services are the right place for it.