An heirloom strain is a cannabis strain that has been cultivated to keep the seed genetics preserved and passed down unchanged for generations. Heirloom strains have not been crossbred or genetically manipulated and usually display characteristics of pure Indica or pure Sativa instead of any type of hybrid.
Heirloom strains differ from landrace strains in that landraces naturally occur in specific regions of the world and are considered to be native with genetics that have not been crossbred with other strains. An heirloom strain is basically a landrace strain that has been taken somewhere else and grown elsewhere, maintaining the same plant genetics for decades.