solar sail

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

• noun A saillike device made of lightweight reflective material, attached to a spacecraft to harness the radiation pressure of the sun's light and solar wind for propulsion.

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

• noun astronautics A sail-like structure on a spacecraft consisting of enormous ultrathin mirrors, propulsion being achieved by the radiation pressure of light from a star or laser.