In Java, an abstract class is a class that cannot be instantiated directly, but is instead intended to be subclassed by other classes. Abstract classes are used to define a common interface for a set of related classes, and to provide some default implementations that can be used by those subclasses.

An abstract class is declared using the "abstract" keyword in its class declaration. Abstract classes can have abstract methods, which are declared without an implementation, as well as concrete methods, which are implemented in the abstract class. Subclasses of an abstract class must provide implementations for any abstract methods that are declared in the abstract class.

Here is an example of an abstract class in Java:

public abstract class Shape {
    private String color;

    public Shape(String color) {
        this.color = color;
    }

    public String getColor() {
        return color;
    }

    public abstract double getArea();
}

In this example, the Shape class has a private field for the color of the shape, a constructor that sets the color, and a getColor() method that returns the color. The Shape class also has an abstract method, getArea(), which must be implemented by any subclasses of the Shape class. Subclasses of Shape might include classes like Circle, Rectangle, and Triangle, which would each implement their own versions of the getArea() method.