Aperture is the opening in the shell of a gastropod, through which the snail emerges. This opening can be closed, in most cases, by an operculum.

The aperture is generally elliptical or oval, but its upper part (the parietal wall) can have a truncated appearance. However, the aperture can also have many other forms: round, semicircular, trilobate or auriculate. This shape corresponds roughly to the cross-section of the last body whorl.

An aperture has three walls:

• parietal: wall of the penultimate whorl
• columellar: next to the columella
• palatal: outer free wall of the last whorl

The aperture is surrounded, except at the top, by the aperture margin.