Increased or multiplied by fifteen times; having fifteen times the quantity, size, or degree.
From fifteen plus -fold (suffix meaning times, as in tenfold). This follows the standard English pattern for creating multiplication descriptors.
When something increases 'fifteenfold,' it's dramatic—like a one-dollar investment becoming fifteen dollars—but the number 15 is mathematically less common than 10 or 100, so you hear this word less often.
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