MPreg

The "what" of this definition is very simple: MPreg = male pregnancy, usually the result of bad slash. The "why" is something that many PPC agents still struggle to understand. Why do fanfiction writers feel that it will improve their stories to write a physically impossible occurrence between two men? Do they not have a basic understanding of biology? Unless that universe has sentient seahorses, MPreg is always absurd and is always a major pain for agents on a mission.

(There is one canonical instance of MPreg, in an episode of Star Trek: Enterprise, but it was entirely due to the natural biology of an alien species, and the unfortunate pregnant man was not actually the child's father, just a host for the alien embryo. Also, unlike most fanfiction MPregs, there was no slash relationship: the female alien accidentally impregnanted the human man simply by putting both their hands into a bowl of (what was essentially) pebbles at the same time. This does not mean that MPreg is canonical in Star Trek.)

The Cure
Sadly, fangirls who believe it's "cute" to have Jack Sparrow impregnated by Legolas will write what they will, and if there's nothing that logic and common sense can do to stop them, then our chances are slim to nil. Fortunately, the Department of Sufficiently Advanced Technology has come up with a remedy for such an event: the Embryo Extraction Kit. This resembles the de-bugger used in The Matrix, but hooks up to a life-supporting receptacle, depositing the embryo into a friendlier environment than some poor canonical's intestinal tract. Since the child of Jack Sparrow and Legolas could never exist in canon, the fetuses are under the care of Medical Department until birth, after which they are placed in Canon Nursery School.

Good Mpreg: Possible?
In a word: Yes. Just like good scriptfic or smutfic, good mpreg fic does exist, but it's much rarer than the bad sort and takes a great deal more skill to write properly. Signs of a good mpreg fic include:
 * Explaining why the male is pregnant, and how this could work. Maybe there's technology or magic that can make it happen. Maybe they're part of a species where males do occasionally get pregnant. Maybe there's shape-shifting involved, or the male in question is transgender and still has the required uterus and baby-making accessories.
 * The story may take place in an AUverse where all males can get pregnant, but this requires the usual careful crafting of the AU itself, taking into account what might be different if this crucial gender difference had never existed.
 * Making sure that the male(s) in question stay in character. Turning one into a hormonal pregnant person and the other into a fluffy-puppy caring nursemaid is right out.
 * Having people react realistically to the problem of the pregnant man. If this takes place in the Real World, for example, most people would assume the man had a tumor or was simply getting fat.
 * Where, exactly, does the baby emerge? Clue: The human anus does not work that way. Well-written mpreg will generally do something like avoid the issue with a C-section or else use magical/technological means to provide female anatomy that can serve the purpose.
 * Could this story be done with an adopted child? If so, there may be no reason to have an mpreg fic at all.

The Division of Mpreg
The Department of Bad Slash's Division of Mpreg takes care of administering the above cure, and Bad Slash agents in general may often run across and have to fix Mpreg in their own fics.

See also Squick.