MILAB stands for Military Abduction. The phenomenon was first investigated by Dr. Helmut Lammer of the Austrian Space Research Institute. In a number of cases abductees, and sometimes UFO witnesses, are subsequently abducted by humans in military uniforms. (For this reason, Melinda Leslie started calling them RE-ABs). Nearly always unmarked helicopters are involved. The victims are usually drugged or sometimes disoriented by a strong electromagnetic field. During their military abduction most of them receive an implant, usually behind the ear. MILABs seem to be a part of a wider operation, aimed at monitoring and abducting abductees in order to keep a lid on the alien abduction phenomenon.
MILABs are different from regular alien abductions. For starters, they are limited to the USA (though I am aware of claims about some cases - at present unconfirmed - in Canada, the UK and Australia). Furthermore, MILABs involve "Activity of dark unmarked helicopters, the appearance of strange vans or buses outside the houses of abductees, exposure to disorienting electromagnetic fields, drugging, transport with a helicopter, bus or truck to an unknown building or an underground military facility. Usually there are physical aftereffects, like grogginess and sometimes nausea after the kidnappings. There is also a difference when the abductors appear. In most UFO-abduction cases, the beings appear through a closed window, wall, or the abductee feels a strange presence in the room. Most abductees report that they are paralysed from the mental power of the alien beings. At MILABs the abductee reports that the kidnappers give him or her a shot with a syringe. It is interesting, that MILAB-abductees report that they are examined from human doctors in rectangular rooms and not in round sterile rooms -- as in descriptions of UFO-abductees [4, 5, 6, 7]. The described rooms, halls and furniture are similar to terrestrial hospital rooms, laboratories or research facilities and have nothing to do with UFO-furniture." (Helmut Lammer, "Military involvement in Abductions," 16 October 1996, www.abduct.com/features/f04.php).
MILABs are controversial. Some authors, like Greer and Aartsen, believe that the majority - if not all - of abductions are actually performed by the Military, and 'Programmed Life Forms.' There is however little to no evidence available to support those claims. And in my personal experience investigating abduction cases, the MILABs are the exceptions, not the rule.
In his article Lammer speculates there are three main reasons why MILABS take place: mind control experiments, genetic research, and monitoring extraterrestrial abductions.