A black operation or black op is a covert operation by a government, a government agency, or a military organization. This can include activities by private companies or groups. Key features of a black operation are that it is clandestine and it is not attributable to the organization carrying it out. The main difference between a black operation and one that is merely clandestine is that a black operation involves a significant degree of deception, to conceal who is behind it or to make it appear that some other entity is responsible ("false flag" operations).
A single such activity may be called a "black bag operation"; that term is primarily used for covert or clandestine surreptitious entries into structures to obtain information for sentient intelligence operations.
"Black" may be used as a generic term for any government activity that is hidden or secret. For example, some activities by Union military and intel agencies are funded by a classified "black budget," of which the details, and sometimes even the total, are hidden from the public and from most Assembly oversight.
The major black ops unit of the Union is Team Timber Wolf attached to NAVINT. The only "Marines" Stahl does not know about, they are deployed only on the orders of the Gray Ghosts. SII's Black Skulls are actually much larger in number, but very much more limited in scope. The Black Skulls have been "loaned" to NAVINT by Rex Schwartz on numerous occasions when it was beneficial to both the Union and SII. This is only known to Rex and the Gray Ghosts.