Rationalization
Etymology
From "rational" (Latin "ratio," reckoning, reason) plus "-ization." Sigmund Freud and his circle adopted it in the early 20th century as a name for a defense mechanism: dressing an uncomfortable impulse in respectable logic.
noun · /rash-uh-nuh-luh-ZAY-shun/
The invention of a plausible but false reason to justify a behavior whose real motive is less flattering.
What it names
The after-the-fact story we tell ourselves — the comforting reason that hides the actual one.
Example
"I deserved a treat" was pure rationalization for a purchase he already regretted.
Related idea
Motivated reasoning is the broader process of thinking toward a wanted conclusion; rationalization is its output — the tidy justification produced once the deed is done.