hush money

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

• noun A bribe paid to keep something secret.

from The Century Dictionary.

• noun A bribe to procure silence; money paid to prevent disclosure or exposure.

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

• noun A bribe to maintain secrecy (to prevent bad publicity or to prevent the discovery of a crime).

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

• noun a bribe paid to someone to insure that something is kept secret