Plain English Meaning
The tire rubber has been heated so much that it has gone soft and sticky again, almost like it was before it was originally hardened during manufacture.
Full Definition
A condition in which rubber has been returned to a soft, sticky, unvulcanized form by intense localized heat. In reverted rubber hydroplaning, friction between a locked tire and a wet runway generates enough heat to steam-clean and soften the tire's tread surface, leaving the rubber in this gummy, pre-vulcanized condition rather than its original cured, hardened form.
Context Anchor
Seen in discussions of reverted rubber hydroplaning during landing rollouts, especially after hard braking on a wet runway.
Derivation
Curing is the manufacturing process that hardens raw rubber into its tough, finished form (vulcanization). 'Uncured' means that hardening has been undone — the rubber has reverted toward its original soft .