By David Ingram WASHINGTON (Reuters) – If U.S. antitrust enforcers decide to challenge the proposed $45 billion merger of Comcast Corp and Time Warner Cable Inc, it may be because of an idea with a funny-sounding name that has been gaining currency in government offices. The idea is monopsony power, the mirror image of the better-known monopoly power but a concept that is just as old. A monopoly is one seller with many buyers, while a monopsony (pronounced muh-NOP-suh-nee) is one buyer with many sellers. The U.S. Justice Departments Antitrust Division is all but certain to examine the potential monopsony power, or buying power, that a combined Comcast and Time Warner Cable would have over media companies that provide TV programming, according to lawyers with expertise in antitrust law.