Oatmeal is a preparation of oats that have been de-husked, steamed, and flattened, or a coarse flour of hulled oat grains (groats) that have either been milled (ground), rolled, or steel-cut. Ground oats are also called white oats. Steel-cut oats are known as coarse oatmeal, Irish oatmeal, or pinhead oats. Rolled oats were traditionally thick old-fashioned oats, but can be made thinner or smaller, and may be categorized as quick oatmeal or instant oatmeal depending on the cooking time required, which is determined by the size of the oats and the amount of precooking.

Rolled oats are a type of lightly processed whole-grain food. They are made from oat groats that have been dehusked and steamed, before being rolled into flat flakes under heavy rollers and then stabilized by being lightly toasted.