urushiol

noun

1. (countable, uncountable)An oil found in plants of the family Anacardiaceae, causing an allergic skin rash on contact; consists of a variable mixture of several related organic compounds.

   “All three produce an oil, called urushiol, that is a potent allergen. Urushiol is most abundant in the leaves and stems of these plants...”

   “Poison ivy vines also contain the irritating oil urushiol, so it is important to be able to recognize them in winter.”

   “Upon exposure to air, urushiol can turn dark brown, which makes it easier to spot.”