This term refers to a genetically-transmitted retrovirus (in other words, the virus is imbedded in the organism's DNA and is passed on to its offspring) that can't replicate in the host species that is harboring it but which can infect and replicate in other species.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.