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 are mainly people with a Celtic and/or Native American ancestry, but no research has ever been done to back up those claims. On the contrary, researchers that have been contacted contradicted those claims. (Personal research).
A recent survey with regard to 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 a number of problems with the survey however. Follow-up research is needed. (http://www.icar1.com/Contactee-Blood-types.html)