noun
the quality or state of being relentless; unyielding or determined in purpose or effort
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The relentlessness of the team's pursuit of excellence is truly impressive.
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Her relentlessness in chasing her dreams is inspiring to everyone around her.
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I can't believe the relentlessness of that salesperson, they just won't take no for an answer.
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The relentlessness of time never stops, always moving forward without pause.