What Does it Mean to be "Middle Class?"

 I think being middle class means that a person/family's income isn't exactly rich, and isn't exactly poor. A person's class is based on their income, in my opinion. Because of this, if a person gets a job with a higher paying income, then the can change to upper class. In the same way, if a person starts to lose money, or they become unemployed for a long period of time, then they change to a lower class. Just because you are born into a certain class doesn't necessarily mean that you'll stay in that class for the rest of your life.