Arendt's book introduced the expression and concept "the banality of evil". Her thesis is that Eichmann was not a fanatic or sociopath, but an extremely average person who relied on clichéd defenses rather than thinking for himself and was motivated by professional promotion rather than ideology. Banality, in this sense, is not that Eichmann's actions were ordinary, or that there is a potential Eichmann in all of us, but that his actions were motivated by a sort of stupidity which was wholly unexceptional. She never denied that Eichmann was an anti-semite, nor that he was fully responsible for his actions, but argued that these characteristics were secondary to his stupidity.
This concept has been frequently misunderstood. In his 2010 history of the Second World War, Moral Combat, British historian Michael Burleigh calls the expression a "cliché" and gives many documented examples of gratuitous acts of cruelty by those involved in the Holocaust, including Eichmann. Arendt certainly did not disagree about the fact of gratuitous cruelty, but "banality of evil" is unrelated to this question. Similarly, the first attempted rebuttal of Arendt's thesis relied on a misreading of this phrase, claiming Arendt meant that there was nothing exceptional about the Holocaust.