What is this type of method overriding called in Java?

I'm relatively new to Java and I'm using a new API. I came across this method override and I'm not sure what this is called:

public void exampleMethod() {
    Button loginButton = new Button("login"){
       public void onSubmit(){
          //submit code here
       }
    };
}

From what I understand, this is overriding the onSubmit method of the Button class. I've never come across this type of overriding before. Is there a specific name for it? I want to read up more about it but I can't find it. All my searches so far result to regular method overriding by creating a new class, which is what I'm already familiar with.

I'd appreciate if someone could point me in the right direction.

Thanks.