Definition: Perennials are considered to be ornamental plants that are expected to live longer than 2 years. Unlike annual plants that complete their life cycle in 1 growing season and biennials which need 2 growing seasons to mature and go to seed, perennials may go to seed every year and may even die back to the ground at times, but their root systems are very much alive and the plants will continue growing when conditions are right.
This does not mean they live forever. In fact many perennials are considered to be short-lived, lasting only 2-3 years. Rose campion is a short lived perennial, but because it self-seeds so readily, it appears to live much longer.
Not all plants with the ability to be perennial are hardy in all areas. Some can be killed by freezing temperatures, excessively dry conditions or other growing conditions. This is why hardiness zones are so important. Knowing what zone you garden in will allow you to determine what plants will survive in your area.
The term perennial is most often used for plants with showy flowers, but plants like ornamental grasses, tropicals and other plants that have their own categories may also be perennial. The term herbaceous perennial further narrows the group to plants with soft, green stems that die back to the ground in colder climates. Perennial trees and shrubs would be considered woody or non-herbaceous perennials.
Examples: Many of the perennial plants I admire are not hardy in my Zone 4 garden.