An heirloom plant is an open-pollinated cultivar that was commonly grown during earlier periods in human history, but which is not used in modern large-scale agriculture.
Since most popular heirloom plants are vegetables, the term heirloom vegetable is often used instead.
Before the industrialization of agriculture, a much wider variety of plant foods was grown for human consumption.
In modern agriculture in the Industrialized World, most food crops are now grown in large, monocultural plots owned by corporations.
In order to maximize consistency, few varieties of each type of crop are grown.
These varieties are often selected for their productivity, their ability to withstand the long trips to supermarkets, or their tolerance to drought, frost, or pesticides.
Nutrition, flavor, and variety are frequently secondary and tertiary concerns, if at all a concern.
Heirloom gardening can be seen as a reaction against this trend.