Reverse racism is often called out whenever a white person feels discriminated against because of their race. The problem with this notion is that it focuses purely on racism as an interpersonal construct, rather than a systemic problem. In other words, when a white person feels they are experiencing reverse racism, they are probably facing prejudice (as opposed to systemic discrimination) if anything at all.
For example, a white child in a majority black class in one of the many segregated schools in New York City may be bullied for not being black. This would be racial prejudice, but without the institutional power to make it more harmful. Yes, it may harm the individual child in the classroom, but when that child leaves the school, grows up and looks for opportunities to flourish, they will be more likely to get a good job and not be criminalised by the police because of the colour of their skin. The movies that the child watches, the advertisement they see, the history they learn will all reflect whiteness as the norm. The social and institutional power advantages that white child in the long run, while the bullying based on racial prejudice may harm them on an interpersonal level.
The difference is that systemic racism tends to privilege white people, especially in the UK. Calling out reverse racism is short-sighted and ignorant of systems of power that are historically and presently intertwined with individual racial experiences.
So inviting only people of colour to an event is not reverse racism against white people. Discussing what white culture might look like is not reverse racism. Saying that Black Lives Matter is not reverse racism because it is a response to the mainstream narratives that constantly undermine the value of black people and favour white people’s lives. In contrast, shouting out “White Power” is racism because it is said in a context of white supremacy, where white people have historically been, and still are, privileged by the colour of their skin.
In a nutshell, it could be said that racism = power + prejudice.
And reverse racism = prejudice – power. Or even just "perceived prejudice;" a manifestation of white fragility (in saying that "ALL Lives Matter” for instance).