Exaptation
Etymology
Coined in 1982 by paleontologists Stephen Jay Gould and Elisabeth Vrba, from Latin "ex-" ("out of") and "aptus" ("fitted"), deliberately modeled on "adaptation." Feathers, likely first evolved for warmth and only later used for flight, are the textbook case.
noun · /eks-ap-TAY-shun/
A trait that evolved for one purpose but later got co-opted for a completely different use.
What it names
A sharp concept for repurposing — reaching for it whenever something built for one job turns out to be perfect for another it was never designed for.
Example
The messaging app was built for coordinating deliveries; its exaptation into a social network was a happy accident.
Related idea
An adaptation is shaped by selection for its current function; an exaptation is a trait pressed into a new role it was never selected for — evolution improvising with what it already has.