Cover up
A Cover up is when a group of people (usually a government or secret service) systemically lies, using plausible deniability and disinformation, such that the public never finds out the truth about the issue. Cover ups are usually used for highly controversial issues which would either damage the government's credibility or else would cause widespread panic if the truth was known. The term cover up can also be used to refer to individual actions that are not of national importance, for example businesses covering up immoral or illegal action, or individuals on the internet covering up doing something reprehensible.
Cover ups are often used interchangeably with the term conspiracy theory, although they are also often used to mean entirely different things.
A cover up does not require for a conspiracy theory to exist, although a conspiracy theory may exist as part of the cover up.
Whilst a conspiracy theory requires that something immoral was done as part of the conspiracy theory, a cover up does not. Most cover ups are done for understandable reasons - to either avoid government embarrassment or else to stop nationwide or worldwide panic.
Cover ups deal with facts, whilst conspiracy theories are theories about what happens. Most cover ups do have conspiracy theories to help to explain the cover up, but the cover up itself is not a conspiracy theory.
Most major controversial issues that have a nationwide or worldwide impact have some level of a cover up as a part of the issue.