An impurity is something unwanted mixed into something else, making it less pure or perfect. When you fill a glass with water but a speck of dirt floats in it, that dirt is an impurity. When gold miners extract gold from rock, they have to remove impurities like other metals and minerals to get pure gold.

The word comes up often in science and manufacturing. Scientists testing medicine need to check for impurities that might make it unsafe. Computer chip makers work in ultra-clean rooms because even a single dust particle as an impurity could ruin a tiny electronic component. Steelmakers add certain elements to iron and carefully control which impurities stay and which get removed to create steel with exactly the properties they need.

Impurity can also describe something that corrupts or taints an abstract quality. In older literature, people sometimes wrote about “moral impurity” or “impure thoughts,” meaning ideas or actions that contaminated someone's character or intentions. Today we might say someone's motives weren't pure if they claimed to help while secretly seeking personal gain. That selfish motive would be the impurity in their otherwise generous act.

The opposite of impurity is purity. Pure gold contains no other metals. Pure water contains only water molecules. When chemists measure purity, they're checking what percentage is the desired substance versus unwanted impurities.