virtual particle

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

• noun A subatomic particle that is allowed by Heisenberg's uncertainty principle to exist for a short period of time, but whose continued existence would violate the principle of conservation of energy.

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

• noun physics a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum