A meal replacement is a drink, bar, soup, etc. intended as a substitute for a solid food meal, usually with controlled quantities of calories and nutrients for purposes of weight loss. Some drinks are in the form of a health shake. Medically prescribed meal replacement drinks, which include required vitamins and minerals, include Optifast and HMR. Bodybuilders sometimes use meal replacements, not formulated for weight loss, to save food preparation time when they are eating 5 to 6 meals a day.

A meal replacement must provide between 200 and 400 food calories of energy, of which not more than 30% from fat, and not less than specified amounts for various vitamins and minerals. Labelling information is prescribed, and packaging must provide information such as a statement that the product should not be used for more than three weeks without medical advice. This protects users of meal replacements without other food from inadvertent malnutrition.

In the United States, the term "meal replacement is not defined in federal Food and Drug Administration regulations, but generally refers to a calorie-controlled, prepackaged product in the form of a bar or beverage (ready to drink or powder), that replaces a regular meal. Meal-replacement products usually provide 200 to 250 calories per serving, are fortified with more than 20 vitamins and minerals at “good” or “excellent source” levels and often bear nutrient content claims, such as percent fat free and reduced sugar." Meal replacement products can be regulated as conventional/functional foods.