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.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.