Loosely defined, “perennial” refers to those flowering plants that return from the rootstock year after year. Alan Armitage quotes Henry Beach with this gem:
"A perennial is any plant that, had it lived, would have bloomed year after year."
But, seriously…
They depend largely upon winter conditions for definition because a plant that will happily survive a Maryland winter (zone 7) will probably die during a typical winter in southern Maine (zone 5). Some examples: gaura, verbena and rosemary.
So we can define perennials regionally. It is this aspect that makes relocated gardeners anxious when planning new gardens—is it hardy here in my new locale?
All herbaceous perennials have fleshy, green stems and require a rest in winter. Many die back to the crown following a killing frost (some examples: daylilies, columbine and iris). They may be evergreen (some examples: dianthus, a few fern and heuchera). But some have a woody stem and persist through the seasons (some examples: lavender, tree peony and nepeta).
Generally undemanding, perennials naturally prefer a compost enriched, well-drained soil. Rock garden plants will need the addition of coarse builders sand or other rocky material to be happy. The addition of compost each year will generally take care of perennials nutritionally. Over-feeding can and does occur. Not a good idea.
After they are in the ground for a season, many are amazingly drought tolerant, although regular deep watering is advised. Their roots will grow deep into the soil with this method and thus improve drought tolerance. A greater risk to their health comes from being over watered or being planted where the roots remain wet for long periods—especially in winter.
Diseases are rare—although airborne fungi may attack some vulnerable groups, producing mildew (some examples: summer phlox, beebalm and delphinium). Prevention is possible with the use of fungicides or the selection of resistant varieties.
This endlessly fascinating, enormous group of plants can and does keep gardeners trying new plants for years and years. Some have said they would need another lifetime to do them justice.