To give something a new meaning or change what it means, or to describe something in a completely different way.
From 're-' (again) plus 'define' (from Latin 'definire'—to set limits). The prefix re- shows the word means to do defining over again.
Every generation redefines words—'cool' used to mean cold, then it meant calm, then it meant awesome. And 'work' now means something totally different in the gig economy era than it did for your grandparents.
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