meadowsweet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various shrubs of the genus Spiraea of the rose family, especially S. alba of North America, having tapered clusters of small white or light pink flowers.
• noun Any of various perennial herbs of the genus Filipendula of the rose family.

from The Century Dictionary.

• noun Any plant of the genus Spiræa, primarily S. Ulmaria of the Old World; in the United States more especially S. salicifolia.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) The name of several plants of the genus Spiræa, especially the white- or pink-flowered Spiræa salicifolia, a low European and American shrub, and the herbaceous Spiræa Ulmaria, which has fragrant white flowers in compound cymes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany A Eurasian perennial flowering plant of Rosaceae family, Filipendula ulmaria.
• noun botany A common name for the genus Spiraea of the Rosaceae family, native to the temperate Northern Hemisphere and consisting of about 80-100 species of shrubs.