NEACP is a U.S. military
abbreviation/acronym. It is an acronym solely by means of the fact that it is usually pronounced 'Kneecap' despite the mangling that implies. The abbreviation stands for National Emergency Airborne Command Post.
NEACP is one of several children of the Cold War. One problem which consumed a lot of planners' worrying time during said time was that of the problem of fighting a nuclear war (much less running a country) when your national leadership (known as the NCA) were either dead or even just running frantically around the country trying not to get bombed. Several alternate command facilities were designed, with one purpose in mind - to allow any NCA members who reached them the ability to survive attack, and from the command post command counterstrikes and general military actions in response - as well as, one hoped, call the whole thing off should anyone decide it was 'over.'
In addition to NEACP, there was the ANMCC (Alternate National Military Command Center), the NECPA (National Emergency Command Post Afloat, aboard a cruiser stationed at Newport News usually) and of course the NMCC beneath the Pentagon. Of these, only NEACP and the NMCC survive; the ANMCC at Site R has been deactivated as of 1997 (and the Congressional equivalent the Greenbrier can, in fact, be rented out for parties!) and the NECPA was deactivated in 1970 under the assumption that by the time the NCA got to said cruiser, it, too, would likely be toast. These were all components of a larger system, which, of course, boasts lots of letters in its name - the WWMCCS, or World-Wide Military Command and Control System (whew). Oh yes - the knowledgeable Jurph has reminded me that since 1994, this aircraft has been known as NAOC, or National Airborne Operations Center ('Operations' being much more buzzword compliant than 'command,' of course). Note: sekicho informs me that the driver behind the name change was the FEMA.
NEACP was intended to serve as a 24-hour alternate in times of extreme crisis - the President could either move to NEACP or move the Vice-President (or other responsible party) there in case he was unable to make it away from Washington. It consisted, for much of its service life, of a fleet of four E4-B aircraft, heavily modified with extra communications and support gear as well as air-to-air refueling capability. Air Force One has some (but not all) of the facilities to serve as NEACP in time of crisis, as well. Of these four, one aircraft is constantly staffed 'just in case' and one will meander around 'nearby' when the POTUS and Air Force One are travelling.
Anecdote: this airplane was the cause of one of my favorite examples of Dan Quayle's complete and utter stupidity. NEACP had originally been stationed at Andrews Air Force Base in order to facilitate quick access by the President. However, in the 1980s, it was decided that it should be moved away from DC, since the President would be aboard helicopters in any case; that way it would be more likely to survive, and was less vulnerable to sabotage, etc. While it couldn't remain so forever, the location to which it was being moved was kept secret in official media, for obvious reasons.
The next day, Quayle (then a freshman Senator) gave a press conference to state how happy he was that NEACP was being relocated to his home state of Indiana, and that he was sure there would be many jobs and contracts coming their way due to his unstinting effort to defend his state's interests.