In Java, a marker annotation is an annotation that doesn't have any elements or parameters. It is simply used to mark a piece of code as having some special property or characteristic. Marker annotations are also known as "empty annotations" or "flag annotations".

The most common example of a marker annotation in Java is the @Deprecated annotation, which is used to mark a class, method, or field as deprecated and no longer recommended for use. Here's an example of the @Deprecated annotation being used to mark a method as deprecated:

@Deprecated
public void oldMethod() {
    // method body
}

In this example, the oldMethod() method is marked as deprecated using the @Deprecated annotation. The annotation doesn't have any parameters or elements, but it tells other developers that this method should no longer be used.

Another example of a marker annotation is the @Override annotation, which is used to indicate that a method in a subclass is intended to override a method in its superclass. The @Override annotation doesn't have any parameters, but it tells the compiler that the annotated method should override a method in the superclass.

public class SubClass extends SuperClass {
    @Override
    public void overriddenMethod() {
        // method body
    }
}

In this example, the overriddenMethod() method in the SubClass is marked with the @Override annotation, which tells the compiler that this method should override the overriddenMethod() method in the SuperClass.

Marker annotations are also used to provide additional information to tools and frameworks that process Java code, such as code analyzers or code generators. They can be used to specify how the code should be processed, or to provide hints to the tools about the expected behavior of the code.