Ibram X Kendi, in his foundational work How to be an Antiracist, defines an antiracist as "one who is supporting an antiracist policy through their actions or expressing an antiracist idea." He goes on to define an antiracist policy as "any measure that produces or sustains racial equity between racial groups."
Finally, Kendi argues "To be antiracist is a radical choice in the face of this history, requiring a radical reorientation of our consciousness" and that "being an antiracist requires persistent self-awareness, constant self-criticism, and regular self-examination."
Over the last few decades, our understanding of racism has expanded beyond dictionary or textbook definitions. A common phrase in social justice work is racism is prejudice + power (first introduced by social scientist Patricia Bidol-Padva). Combining the concepts of prejudice and power points out the mechanisms by which racism leads to different consequences for different groups.
Anti-Blackness or anti-Black racism is a term to highlight the ways in which Black people in particular experience racism. While many racial and ethnic groups experience racism or prejudice, the term anti-Black racism underscores the unique experiences of racism for Black people.