Perhaps I should have been more clear: the definition is fuzzy, but Adria's original tweet is not inherently wrong -- it just lacks context and adequate explanation.

I'm definitely not an expert in this, but I'm speaking from reading books on sociology and dominant group psychology. It would take more time for me to look up the original sources, but Wikipedia does agree with me (and they also agree that the definition varies!).

http://en.wikipedia.org/wiki/Racism#Definitions

"David Wellman has defined racism as “culturally sanctioned beliefs, which, regardless of intentions involved, defend the advantages whites have because of the subordinated position of racial minorities”."

"Sociologists Noël A. Cazenave and Darlene Alvarez Maddern define racism as “...a highly organized system of 'race'-based group privilege that operates at every level of society and is held together by a sophisticated ideology of color/'race' supremacy."

Both of these definitions involve power as the main mechanism for preserving prejudice, do they not?

These definitions aren't scientific but politicized. They come from activists with a PhD. They just want to "prove" that capitalism is imperialist and racist.

Quite. The purpose of twisting the definition of 'racism' is to use the emotional power of the term to support an unrelated political agenda. The irony is that the academic definition of 'racism' is only rhetorically useful as long as the common definition remains dominant, otherwise all resonance is lost: Jim Crow and slavery cause universal revulsion; industrious individuals accruing wealth, not so much.