lilac, any plant of the genus Syringa, deciduous Old World shrubs or small trees of the family Oleaceae (olive family), widely cultivated as ornamentals. Since colonial days, the common lilac has been in America one of the best loved of the flowering shrubs, meriting its favor by its cone-shaped masses of lavender or white flowers, its fragrance, and its ease of cultivation. Some cities (e.g., Rochester, N.Y.) have lilac festivals. The purple flower clusters are the floral emblem of New Hampshire. From this old-fashioned common lilac ( S. vulgaris ) and others, many hybrids have been developed with variations in form (such as double flowers) and in color (such as rosy pink and white). These hybrids, which may lack the fragrance of the common lilac, are often called French lilacs because much of the pioneer hybridizing was done in France. The most famous use of the lilac in poetry is Whitman's elegy on Lincoln, "When Lilacs Last in the Dooryard Bloom'd." The lilac should not be confused with the unrelated mock orange (of the saxifrage family), which is sometimes also called syringa; both plants are sometimes called pipe tree. Lilacs are classified in the division Magnoliophyta, class Magnoliopsida, order Scrophulariales, family Oleaceae.
See D. Wyman, Shrubs and Vines for American Gardens (rev. ed. 1969).