Affiliate parasitism refers to the process by which some affiliates earn commissions they don’t deserve because they don’t really influence or provoke the purchase decision.
Usually parasitic affiliates "intervene" and drop their affiliate cookies just before a web user visits the merchant website and makes the purchase. They benefit from "the last cookie counts" principle.
By dropping their cookies, they get credit for a commission which theoritically is due to the rightful affiliate that drove the sale or from the merchant’s own marketing efforts.
To consider an affiliate as a parasitic one is all a matter of point of view and of course, parasitism is a term used mainly by "legetimate" affiliates. Some cases seem to be clear, others are not.
Many websites or softwares (also called parasitwares) may be categorized as practising affiliate parasitism, see parasitic affiliate for more details.