For foods and beverages, U.S. FDA regulation 21 CFR 101.22 defines and provides guidance for essential oils labeling. Section 182.20 list Essential oils that are GRAS (generally recognized as safe). It lists Lemon- Citrus limon (L.) Burm. f. As the variety that is GRAS.
Lemon Essential Oil can be labeled as Natural Flavor or Natural Flavoring according to Section 101.22
“The term natural flavor or natural flavoring means the essential oil , oleoresin, essence or extractive, protein hydrolysate, distillate, or any product of roasting, heating or enzymolysis, which contains the flavoring constituents derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products, or fermentation products thereof, whose significant function in food is flavoring rather than nutritional. Natural flavors include the natural essence or extractives obtained from plants listed in 182.10, 182.20, 182.40, and 182.50 and part 184 of this chapter, and the substances listed in 172.510 of this chapter.”