The air-filled pocket between the white and shell at the large end of the egg.
When an egg is newly laid, it is about 105ºF (41ºC) and has either no air cell or a very small one. As the egg cools, the liquid contents contract more than the shell and the inner shell membrane separates from the outer shell membrane to form the air cell. As the egg ages, moisture and carbon dioxide leave through the pores of the shell, air enters to replace them and the air cell becomes larger. The flattened end of a peeled, hard-boiled egg shows you where the air cell once was.
The formation of the air cell and the separation of the shell membranes are the reason that a slightly older egg is easier to peel after hard-boiling. Storing eggs upright in their cartons in the refrigerator helps to keep their air cells in place and maintain egg quality.
Although the air cell usually forms in the large end of the egg, it occasionally moves freely toward the uppermost point of the egg as the egg is rotated. It is then called a free or floating air cell. If the main air cell ruptures, resulting in one or more small separate air bubbles floating beneath the main air cell, it is known as a bubbly air cell.
Candlers use the size of the air cell as one basis for determining grade.