This another one of the words most mentioned in the Quran and is a central concept of Islam. I have never seen a good translation of it! What the word means is the association of others with God in worship. What English word or short phrase can convey the same meaning?
"Disbelief" is an incorrect translation, because God tells us in the holy Quran that "shirk" and belief can coexist and in fact often do,
I'm inclined to suggest the word "blasphemy," but it is too general where the Arabic word is specific.