As the other answers to this question demonstrate, Harper Lee doesn't limit prejudice in To Kill a Mockingbird to racist prejudice. However, racism is one of the most prominent examples of prejudice in the book, and the theme of racism is most clearly defined during the Tom Robinson trial. During his closing remarks, Atticus very eloquently points out that Tom is obviously not guilty, and that he was unjustly brought to trial. Indeed, Atticus suggests that Tom was only brought to trial because he is a black man living in a racist and prejudiced world. However, in spite of the overwhelming evidence pointing to Tom's innocence, he's still declared guilty by the jury. At this point, Lee is clearly illustrating the prejudiced nature of Maycomb's citizens, as the white jury obviously has preconceived notions that Tom must be guilty simply because he is African American.
According to Mirriam-Webster prejudice is defined as
(1): preconceived judgment or opinion (2) : an adverse opinion or...
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