What is "introgression"

Introgression \In`tro*gres"sion\, n. [L. introgressus, p. p. of introgredi to go in; intro- within + gradi to step, go.] The act of going in; entrance.
--Blount.

n. (context genetics English) The movement of a gene from one species to another

Introgression, also known as introgressive hybridization, in genetics is the movement of a gene ( gene flow) from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Purposeful introgression is a long-term process; it may take many hybrid generations before the backcrossing occurs.

Introgression differs from simple hybridization. Introgression results in a complex mixture of parental genes, while simple hybridization results in a more uniform mixture, which in the first generation will be an even mix of two parental species. Natural introgression does not have human direct interference while the exotic introgression is induced intentionally (as for instance genetically modified organisms) or not (human activities affecting local races of crops or human disturbances such as by introducing weeds).