Note: Information is dated. Do not rely on it.Lilac. A genus of plants belonging to the natural order Oleaceae (which includes the ash, jessamine, olive, etc.). The lilacs are natives of the East; they are shrubs or small trees; the flowers are characterized by a four-cleft corolla, by two stamens, and by a two-valved fruit. Several species are cultivated for ornamental purposes, the common lilac being one of the most extensively cultivated shrubs in Europe.