Chloride is the negatively charged ion (anion) of chlorine, a crucial electrolyte that helps maintain the body's fluid balance and acid-base balance (pH), working alongside other electrolytes like sodium and potassium. It is found in high concentrations in seawater and is also a component of common salt (sodium chloride), which is the primary way the body obtains chloride through food. Blood tests are used to measure chloride levels, as abnormal levels can indicate dehydration, kidney problems, heart failure, or other underlying health conditions.