Short version: an abductee is a person who was involuntarily taken by aliens onto their craft, and subsequently returned.
At the 1992 MIT Conference on abductions, Mark Rodeghier proposed the following definition of abductees:
To qualify as an abductee, a person must be
- taken against his or her will,
- from terrestrial surroundings,
- by non-human beings.
These beings must take a person to
- an enclosed place,
- non-terrestrial in appearance,
- that is assumed or known to be a spacecraft by the witness.
In this place, the person must either be
- subjected to a physical examination,
- engaged in communication, verbal or telepathic,
- or both.
These experiences may be remembered
- consciously or
- through methods of focused concentration, such as hypnosis.
Source: C.D.B. Bryan, Close Encounters of the Fourth Kind.
On the Internet, there are theories that abductees predominantly are people with a Celtic and/or Native American ancestry, but no research has ever been done to back up those claims. On the contrary, surveys done by MUFON and Free refute that claim.
An ICAR survey regarding the blood types of abductees confirmed that all blood types can be found among abductees. The survey does, however, seem to indicate that people with a negative rhesus factor seem to stand a bigger chance of being abducted than people with a positive rhesus factor. There are several problems with the survey, however. Follow-up research is needed. See: http://www.icar1.com/Contactee-Blood-types.html and http://www.exopoliticssouthafrica.org/news/in-depth/8-contactee-blood-type-research