Newton's first law

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

• noun The principle stating that a body at rest will remain at rest, and a body in motion will remain in motion with a constant velocity, unless acted upon by a force.

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

• noun physics The observation that objects in motion tend to stay in motion, and objects at rest tend to stay at rest unless an outside force acts upon them.