The Amniotic Fluid Index (AFI) is a quantitative measure of amniotic fluid volume in a pregnant woman's uterus, used to assess fetal well-being during prenatal care. It is calculated by dividing the uterus into four quadrants, measuring the deepest, unobstructed vertical pocket of fluid in each, and summing these measurements to ensure the fluid level is within a normal range, typically between 8 and 18 cm.