Simple Definition:
A perennial is a plant that lives for more than two years [a plant that lives for a long time]. Perennials typically flower and produce seeds each year [produce flowers and seeds every year]. Examples of perennial plants include roses, daisies, and tulips [flowers like roses, daisies, and tulips that grow back each year].
Very Simple Definition:
A perennial is a type of plant that comes back every year. Examples of perennials are roses, daisies, and tulips.