Generally speaking, an heirloom plant variety is a variety that was grown quite commonly by previous generations but is not used in today's large-scale modern agriculture. Most specifically, the plants must keep their traits uniformly through open pollination. Often the term ‘heirloom’ is categorized by defining age. There are varying opinions, but some say heirlooms are over 100 years old, while others say 50 years old. Most use WWII as a dateline, saying that plant varieties developed before WWII with non-hybridization techniques are considered heirlooms. Some heirloom seed varieties are labeled "heirloom" and some are not - it's more up to you to know about the varieties on the market.