This term is almost exactly what it sounds like, and occurs when a landlord evicts a tenant or refuses to renew their lease in response to a complaint or allegation made by the tenant. These types of evictions are usually illegal, because state law dictates that landlords can evict their tenants only if they have failed to pay or have done something to breach their lease agreement in some way. A retaliatory eviction, however, is usually done in response to a complaint made by the tenant, which is completely within their rights to make. If the tenant withholds rent as leverage to get the landlord to fix whatever they have been making complaints about, that is usually what initiates a retaliatory eviction.
Tenants who have to deal with a retaliatory eviction can often have a hard time fighting back legally. They often have a hard time proving their case in court unless they have outside witnesses, because landlords can present an entirely different cause for eviction.
These evictions often happen in areas that are extremely popular or attractive to the renting population. If a tenant has a complaint that would be expensive or arduous to fix, the landlord often sees it as more cost-effective to evict the tenant, hoping that there will be another tenant who wants to live in the area so badly that they will live with the issue or deal with it on their own. In this case, if the tenant can prove the eviction stemmed from their complaint, a court will likely find the eviction retaliatory and put the blame on the landlord.