I would define dystopia as a society that controls and governs what people have access to, what they look like, and what they are allowed to say.
The first book/movie that I think of when I hear the word dystopia is The Giver. In this book, the knowledge from the past world is erased and the people living in this society only know what has been approved for them to know. Everything from the career that have, the family they are in, and the clothes they wear are chosen for them. They were not told what emotions are so they do not know when they are happy, when they are sad, and when they are doing something wrong. This society is closed off from the rest of the world and the citizens were not allowed to leave.
Although this book describes an extreme dystopia, I think it can be so much simpler and common than we think. When I think of dystopia I think of anything that is the opposite of utopia, an imagined place where everything is perfect. Since almost nothing in the world is perfect, there is a lot of real world examples of things I would consider dystopian. I think an example of a real world dystopian society was the Holocaust.