Having mirror-image molecular structures that are opposite in handedness, like left and right hands that cannot perfectly overlap.
From Greek hetero- (different) + chiral (from cheir, hand). Chemistry term coined in the 20th century to describe molecules with opposite stereochemistry.
This is why some drugs work while their mirror-image versions don't—your body's proteins are also chiral, so they recognize only one hand-version. That's why thalidomide was a tragedy: one mirror-image form was safe, but the other caused severe birth defects.
Complete word intelligence in one call. Free tier — 50 lookups/day.