A plant that lives for two or more years.
A plant that lives for more than two years - i.e. not an annual or biennial.
In horticulture, the term is often used to refer to only herbaceous perennials (those that die back in Autumn and then grow again from the root stock in spring), although technically woody plants such as trees and shrubs are also perennials.
Use this tag for questions about perennials in general. Questions about specific plant families or species should use the appropriate tag for the family or species.