A Mason jar is a molded glass jar used in home canning to preserve food. The jar is mouth has a screw thread on its outer perimeter to accept a metal ring (or band). The band, when screwed down, presses a separate stamped tin plated steel disc shaped lid against the jar is rim. An integral rubber ring on the underside of the lid creates a hermetic seal. The bands and lids usually come with new jars, but they are also sold separately. While the bands are reusable, the lids are intended for single use when canning. Largely supplanted by other products and methods for commercial canning, such as tin cans and plastic containers, glass jars and metal lids are still commonly used in home canning. Mason jars are also called Ball jars, in reference to the Ball Corporation, an early and prolific manufacturer of glass canning jars, fruit jars for a common content, and glass canning jars a generic term reflecting their material and purpose.

Mason jars are everywhere. They line the shelves of artisan grocery stores, filled with homemade blood orange marmalade or pickled fennel, they store your sugar and spices (and everything nices), and they serve as lanterns, candles, and vases at weddings. There are entire cookbooks dedicated to Mason jar salads.