A perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter-lived annuals and biennials. Perennials can be herbaceous, meaning they have little or no woody growth, or they can be trees and shrubs, which are also technically perennials. Herbaceous perennials grow and bloom over the spring and summer, die back every autumn and winter, and then return in the spring from their rootstock or other overwintering structure.

Perennials are typically cold-hardy plants that will return again in the spring. They usually bloom for only one season each year (either spring, summer, or fall), but there are also reblooming and long-blooming perennials. When grown in favorable conditions, perennials often live a long time, but their lifespan is variable, and some may live for only three to five years. Perennials also vary greatly in terms of their care and maintenance. Some may need to be pruned and divided regularly to maintain their vigor and keep them tidy, while others are tough and undemanding, seeming to thrive on neglect.

Perennials that are cultivated include woody plants like fruit trees grown for their edible fruits, shrubs and trees grown as landscaping ornamentals, herbaceous food crops like asparagus, rhubarb, strawberries, and many flowering plants. There are perennials to suit every type of garden, for any aspect or soil type, and they range in size from low-growing herbaceous plants for the front of a border to giants a couple of meters tall for the back. They establish and grow quickly, with most reaching their full size in a few growing seasons. Many perennials make good cut flowers, and the more compact varieties are excellent for growing in pots.