an object that has been passed down through a family for many generations;
e.g. This necklace is a precious heirloom that has been passed down from mother to daughter in our family.
(noun)
(American) a variant of a plant or a breed of an animal that is the result of individual cultivation rather than nature/evolution or large-scale agriculture;
e.g. I grow heirloom tomatoes in my garden and put them in my salads.