means Lasting or recurring year after year, especially a plant that lives through many growing seasons, or by extension anything enduring or constantly recurring.

from From Latin perennis, 'lasting through the year,' built from per- ('through') plus annus ('year') — the same annus that gives us annual and anniversary. So a perennial is, quite literally, a thing that goes 'through the years.' The -ial ending arrived later in English, smoothing the Latin into something an English mouth could hold.