Dominant culture refers to the values, norms and behaviors of the group with the most power in a society, treated as normal for everyone. In the US, that has generally meant white, Christian, middle-class, heterosexual and English-speaking norms.
Those norms show up in everyday rules: what counts as professional hair or clothing, which English is correct, which holidays schools observe, and what a leader looks or sounds like. People outside the dominant culture are often expected to adapt, through code-switching or assimilation, to be taken seriously.
In 1999, Tema Okun described traits she called white supremacy culture, such as perfectionism, urgency and worship of the written word. Her list became widely used in workplace training, and she later cautioned against treating it as a checklist. Laws like the CROWN Act, which bans discrimination based on natural hair, show efforts to stop dominant norms from being enforced as rules.