The title of this story helps to support the theme, which is one of class conflict as we see working class kids in the form of the Greasers ranged against middle and upper-class kids in the form of the Socs. This is the central conflict that drives the novel forward as it is this conflict that leads Johnny and Ponyboy to run from the law after killing a Soc. In one sense, therefore, the title refers to the way in which the Greasers are the outsiders of the novel. As Ponyboy reflects, when there is trouble, it is always the Greasers that are treated unfairly by society and literally are outcasts. The Socs, Ponyboy tells us, "get editorials in the paper for being a public disgrace one day and an asset to society the next." The Greasers are therefore outcasts of society and blamed for every wrong.
However, the significance of the title also deepens when Ponyboy comes to realise how similar Socs and Greasers are. Through his friendship with Cherry and the conversations that he has with other Socs, Ponyboy comes to realise that in many ways they are very similar, although they come from such different backgrounds, and that Socs experience the feeling of being outsiders as well.
Lastly, at the end of the novel, Ponyboy has a picture of the way that the term "outsider" applies to all young people:
I could picture hundreds and hundreds of boys living on the wrong sides of cities, boys with black eyes who jumped at their own shadows. Hundreds of boys who maybe watched sunsets and looked at stars and ached for something better.
The term outsider then also comes to refer to the way in which so many young people are outsiders as they grow up and go through the teenage years. The title therefore is widened in terms of its application as the novel continues.