Huygens' principle

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

• noun The principle that any point on a wave front of light may be regarded as the source of secondary waves and that the surface that is tangent to the secondary waves can be used to determine the future position of the wave front.

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

• noun physics The assumption that each point on a wavefront may be regarded as a source of secondary waves and that the position of the wavefront at a later time is determined by the envelope of these secondary waves; proposed in order to explain the fact that light travels in a straight line and casts sharp shadows.