Racialism is the idea that humanity can be easily divided into well-defined groups (or "races") that are both broad (each "race" should include many humans, such as entire continents) and specific (the categorization method should almost never misidentify someone's "race"). Racialism implies that these races are substantially different from each other and that these racial differences strongly determine the abilities and behavior of individuals and peoples. Essentially, racialism argues that human populations are substantially different from each other to a degree which necessitates biological classification below the species level. In addition, most racialists maintain there is also a "racial hierarchy" of best and worst races.
Early attempts at race classification drew largely on Carl Linnaeus' Systema Naturae (1735), in which he posited five races: the Europeanus, the Africanus, the Americanus, the Asiaticus, and the Monstrosus (made up of mythical creatures). Racialism was virtually unchallenged until the 1960s, when it was shown to be erroneous by genetics.
Simply put, modern racialism is pseudoscientific bullshit. As Lieberman et al 2003 write:
“”The rejection of the race concept […] in the 1960s[ ]was based on the genetic evidence reviewed earlier. Conformity to political correctness was not the cause of these changes[.]