An assay is a scientific test used to determine the presence or quantity of a substance in a sample. This process is commonly used in fields like chemistry, biology, and medicine to analyze materials such as blood, soil, or pharmaceuticals. Assays can help identify the composition of a sample, measure the effectiveness of a drug, or detect contaminants. There are various types of assays, including enzyme-linked immunosorbent assays (ELISA), polymerase chain reaction (PCR), and spectrophotometric assays. Each type has its own specific methods and applications, making assays essential tools for research, diagnostics, and quality control in many industries.