Racism (n.) An over-identification with one's extended family, usually combined with low collective self-esteem, which leads one to dismiss members of other extended families as more or less sub-human.
Racism has been described using long words but basically it depends on two things: the tendency to classify people into "them and us", and the need for self-promotion through the degredation of others. The first appears to be tied to imprinting at an early age, the second due to the fact that many people are pretty messed up. Racism presumably had some evolutionary value in the past. Today, the "them and us" imprinting can be prevented via culturally mixed high-quality education for children from ages 3 to 10.
It is worth noting that racism underlies large parts of Europe's immigration policy, which in broad terms says, "if you ain't white, get out of sight".
Not to be confused with preferential treatment which refers to "job or employment preference given to someone who is of the right race, ethnicity, or gender" as defined in the government's approved list of historically disadvantaged.