white lie

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

• noun An often trivial, diplomatic or well-intentioned untruth.

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

• noun idiomatic A deliberate, untrue statement which does no harm or is intended to produce a favorable result.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an unimportant lie (especially one told to be tactful or polite)