A milligram (mg) is a unit of mass in the metric system, equal to one-thousandth of a gram. It is commonly used to measure small quantities of substances, such as medications, vitamins, and chemicals. For example, a typical aspirin tablet may contain 500 milligrams of active ingredient. Milligrams are essential in fields like pharmacology and nutrition, where precise measurements are crucial for safety and effectiveness. Understanding milligrams helps individuals accurately follow dosage instructions and nutritional guidelines, ensuring proper intake of substances like vitamins and medications.