Describing a material that becomes wider when stretched (rather than narrower, as most materials do), showing an unusual negative Poisson's ratio.
From Greek 'auxesis' (growth) + '-ic' (having the quality of). This modern material science term was coined in the 1970s-80s when researchers discovered materials with counterintuitive expansion properties.
Most materials get thinner when you stretch them—like a rubber band—but auxetic materials get thicker, which is why they're now used in body armor, sports equipment, and aircraft seats to absorb impact better.
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