Any plant that lives longer than two years.
A perennial is recurrent, lasting. Within botany, any plant that lives longer than two years is called a perennial. Known for its ability to though out the seasons the year throws at it. Not all perennials can take the seasons of the world round, so those bred outside of their native region will most often be grown as an annual or biennial. Others, known as herbaceous perennials, will die down for their dormant season—most often winter—giving the illusion of death. But, all they need is for their root structure to remain intact below ground, ready come spring—or any other suitable season—to send up new shoots.
And, that's what many perennials come down to, the ability to keep their root structure intact, and to re-establish plant growth above ground once the weather becomes agreeable. On top of their ability to reproduce by seed or cutting, several perennials have the ability to propagate by expanding their root system. They do so laterally, and this spreading root system will send up shoots near the parent plant. Which can be dug up or cut away and replanted.
Perennial means a plant lives longer than two years, though the actual amount a perennial can cycle through varies wildly from characteristic to characteristic, from genus to genus, and region to region. Perennials can live from a handful of years to several millennia. What eventually causes them to pass is damage and age. Though some plants seem to fall even out of the range of the latter. Senescence—the process of ageing, and therefore breaking down into death—is something a very small amount of organisms are simply not built to do. They can be damaged, physically brought harm to, they can fall prey to negative environmental factors, but ageing or noticeable ageing isn't wired into them. We call this negligible senescence, the absence of measurable signs of ageing. Meaning the organism does not lose in strength, nor increases its chance of dying, or reproductive capacity as it grows older. It simply racks up year after year into a kind of immortality.
When people think of perennials, they'll often think of trees and bushes—plants that form woody growth. But, a great deal of perennials don't fall under either category, so here are a few examples of plants you might not have known to be perennials.
+ The banana family (Musaceae) are herbaceous perennials. Once the top part of the plant is spent, meaning once the fruit has matured, everything above ground will start to die down. It's rhizome (root) structure, however continues on to send new banana suckers up.
Banana plants have a natural tendency towards vegetative propagation (non-sexual reproduction). With each plant capable of sending out suckers, baby banana plants that grow away from the mother plant above ground, though still connected below ground through its root structure.
Within commercial cultivation, these suckers are cut away from the mother, and replanted. This method causes each plant to be a clone of the one before it, and the one before, and the one before, into a long history of human-aided and guided reproduction.
+ Solanum lycopersicum is known as a perennial in its native region of South America. But, ask most people—at least those with a minimum of garden experience—and they'll tell you they're annuals.
Tomato cultivars lean towards two categories: determinate and indeterminate. The first are bred for commercial viability and convenience, but not longevity. They're compact in growth, and predictable in their fruit's maturation process— for fruit that matures during the same period can be picked at the same time.
Whereas, indeterminate tomato plants or vining tomatoes grow rather bushy. It continues to produce new flowers, even as earlier ones have set to fruit and ripen. The changing season and the eventual cold is what kills these kinds of cultivars off. But, under the right circumstances or in their native region, the cold never hits, and they can live as perennials.
AS A SPECIFIC EPITHET
Some plants will carry their perennial nature in their nomenclature. As a specific epithet, perennial is expressed as perennis (m,f) and perenne (neuter).