"GRAS" is an acronym used by federal regulatory agencies to refer to substances that are used as food additives, but are considered to be "Generally Recognized As Safe” when used in accordance with good manufacturing practices.
As set forth in the Federal Food, Drug, and Cosmetic Act (see links below), any substance that is intentionally added to food is considered to be a food additive. All food additives are subject to an extensive premarket review before being approved for use by the Food and Drug Administration (FDA). However, this review requirement can be waived for substances that have been exempted from the definition of a food additive or have been granted GRAS designation. Regulators realize that it is impracticable to compile an exhaustive list of all potential GRAS substances that could be added to food. For example, common food ingredients such as salt, pepper, vinegar, baking powder and monosodium glutamate are considered safe when used as intended and, accordingly, are not listed among GRAS substances.
The following links from Title 21, Chapter 1, Subchapter E of the Code of Federal Regulations provide the most up-to-date information on GRAS: