An heirloom variety vegetable is an open-pollinated cultivar, or cultivated plant, that is not used in large-scale modern agriculture. The strain must be of a certain age to qualify as an heirloom.
Because heirloom varieties are unique seeds that have not been genetically modified or crossed into other strains there are no specific features that they share except that they have usually become accustomed to their local growing area's conditions over the years.
Not only are heirloom varieties usually very tasty--after all, why would a single seed be passed down for so many years?--but they also promote genetic diversity and are not prone to diseases that can wipe out entire varieties of homogenized vegetables.
The exact standards for what officially makes an heirloom variety is under debate. Some say the strain needs to have been grown prior to 1951, which is when hybrid strains were developed, while others insist that the strains need to have originated before 1920.