As with many things in life, one simple definition does not seem to suffice when trying to describe what "heirlooms' or heritage plants are.
The simplest definition would be that heirlooms are 50 years old, but of course this is not always accurate. There are actually considered to be several different categories of heirlooms, just to complicate the matter.
There are heirlooms that are family treasures that have been passed down from generation to generation in a family. Consider for example that fine green beefsteak tomato named Aunt Ruby's German Green-this name was not dreamt up in a marketing session! There really was an Aunt Ruby who saved seed from her favourite tomato and handed it down to her family members. Her name was Ruby Saunders and she lived in Tennessee.
It is in my collection of tomatoes only because she did this and a fine gentleman by the name of Bill Minkey, a collector, passed it along to Seed Savers Exchange. A preserved family treasure, now safe with all tomato seed savers!
An interesting, but controversial category is the "Created Heirloom" category-are they even heirlooms? ( I offer no opinion here on this one!) There is a whole range of tomatoes that have been created by crossing two OP varieties, heirlooms, and growing out the resulting desired fruit until it is stable and OP itself, coming true to type. Perhaps best known in this category is the fabulous Green Zebra , created in 1989, by tomato-potato breeder Tom Wagner. It certainly hasn't been with us 50 years, nor does it have the family history. Fantastic- yes! An heirloom--??
By far the most important thing that heirlooms ARE is open pollinated. The means growers like you and I can save the seed and it will grow true to type ( of course taking precautions to prevent cross pollinating.) Saving seed from a hybrid is a bit of a crap-shoot. Who knows what you will get. But as a seed saver I can save my heirloom seed, and continue to pass it on- ensuring it will exist a bit longer, and another saver will carry on the tradition.
Hope this helps and thanks for asking the question!