A cover-up is an attempt, whether successful or not, to conceal evidence of wrong-doing, error, incompetence, or other embarrassing information. The expression is usually applied to people in authority who abuse their power to avoid or silence criticism. Those who cover up may be those responsible for a misdeed or their allies, or simply people with an interest in silencing criticism.
When a scandal breaks, the discovery of an attempt to cover up is often regarded as even more reprehensible than the original deeds.
The mildest case, not quite a cover-up, is simply to release news which could be embarrassing but is not important enough to guarantee attention at a time when other news is dominating the headlines, or immediately before a holiday or weekend.
Initially a cover-up may require little effort; it will be carried out by those closely involved with the misdeed. Once some hint of the hidden matter starts to become known, the cover-up gradually draws all the top leadership, at least, of an organization into complicity in covering up a misdeed or even crime that may have originally been committed by a few of its members acting independently. This is often regarded as tacit approval of that behaviour.
It is likely that some cover-ups are successful although by definition this cannot be confirmed. Many fail, however, as more and more people are drawn in and the possibility of exposure makes potential accomplices fearful of supporting the cover-up and as loose ends that may never normally have been noticed start to stand out. As it spreads, the cover-up itself creates yet more suspicious circumstances.
The original misdeed being covered may be relatively minor, such as the 'third-rate burglary' which started the Watergate scandal, but the cover-up adds so many additional crimes (obstruction of justice, perjury, payoffs and bribes, in some cases suspicious suicides or outright murder) that the cover-up becomes much more serious than the original crime.
Cover-ups do not necessarily require the active manipulation of facts or circumstances. Arguably the most common form of cover-up is one of non-action. It is the conscious failure to release incriminating information by a third party. This "passive cover-up" is often justified by the motive of not wanting to embarrass the culprit or expose them to criminal prosecution or even the belief that the cover-up is justified by protecting the greater community from scandal. Yet, because of the passive cover-up, the misdeed often goes undiscovered and results in harm to others ensuing from its failure to be discovered. (In Catholic Moral Theology this would be considered the Sin of omission and a Mortal sin)
Real cover-ups are common enough, but any event which is not completely clear is likely to give rise to a thicket of conspiracy theories alleging covering up of sometimes the most weird and unlikely conspiracies.
A cover-up need not involve wrong-doing. In a number of countries the incidence of epidemics of contagious diseases is often covered up, either because of national pride or to forestall panic. This is often counter-productive, allowing the disease to spread unchecked when precautions could be taken. AIDS and avian influenza have been covered up in the first few years of the 21st century. In 2006 China has been more open about avian flu, which has helped in dealing with an epizootic.
Sometimes an apparently simple and low-risk cover-up grows out of control. For example, an employee may take money covertly from his employer to finance something, in the expectation that (s)he will shortly return it with nobody being the wiser; but the money taken is lost, the employee cannot make good, and must dangerously extend the cover-up. Compulsive gamblers, who irrationally think that they will bet the embezzled money, win, return the stake, and keep their winnings are an example. They will typically steal more, still intending to repay it with winnings, until eventually the shortfall can be concealed no longer. The case of derivatives trader Nick Leeson is similar.