An assay is a careful test or analysis to determine what something contains or how pure it is. When prospectors found a rock that might contain gold, they would assay it by testing to see how much actual gold was mixed in with the worthless rock. A chemist might assay a water sample to measure exactly how much lead or other substances it contains.

The word also appears in older writing to mean an attempt or effort to do something difficult. In this sense, to assay a challenging climb means to make a serious attempt at it. You might read in an adventure story that a character “assayed to cross the dangerous bridge” or “made an assay at solving the puzzle.”

Today, the testing meaning is far more common. Scientists assay blood samples to check for diseases, environmental researchers assay soil to find pollution levels, and quality control workers assay products to ensure they meet standards. The key idea is careful measurement: an assay tells you precisely what's there and how much of it you've got, whether you're testing for gold in a mine, iron in someone's blood, or nutrients in farmland soil.