in·tro·gres·sion 
(ĭn′trə-grĕshən)
(ĭn′trə-grĕshən)n.
The movement or spread of alleles of one species into the gene pool of another through repeated backcrossing of interspecific hybrids with members of one of the parental species.
[From Latin intrōgressus, past participle of intrōgredī, to step in : intrō-, intro- + gradī, to step; see ghredh- in the Appendix of Indo-European roots.]
in′tro·gressive (-grĕsĭv) adj.