Black operation

A black operation (black op for short) is a covert operation which is done by a government or military. Black operations are secret and whoever does them does not admit that they ever happened. There are differences between black operations and ones which are just secret. The main difference is that a black operation often uses deception. This deception might be not telling anybody who did the operation. It might also be blaming the operation on someone else ("false flag" operations). 
One example of a black operation is a "black bag operation". A black bag operation is when agents or soldiers secretly go into buildings and collect intelligence. It is known that the Federal Bureau of Investigation, the Central Intelligence Agency, Mossad, MI6 and other intelligence services have done operations like these.
Where the word comes from.
The word "black" can be used for any secret action done by a government. For example, some militaries and intelligence services have classified "black budgets" to carry out black operations. Almost all details about this budget is hidden from the public.