Prejudice is the tendency to falsely jump to conclusions based on things which may have some degree of correlation but which aren't accurate indicators.  "He's black, therefore he's going to try to rape my daughter."  Some black people have raped others, but being black doesn't mean someone is a rapist. On the other hand, hating fats is simply observation.  "They're fat; they must be lazy," isn't a prejudicial jump to conclusion...it's a diagnosis.