What is the "this" keyword?

The this keyword refers to the current instance of the class — the object on which the method is being called. It resolves ambiguity, enables method chaining, and can pass the current object to other methods. Uses of this: (1) Disambiguate field from parameter: class Person { private String name; private int age; public Person(String name, int age) { this.name = name; // this.name = field, name = parameter this.age = age; } }; (2) Call another constructor (constructor chaining): class Rectangle { int width, height, depth; public Rectangle(int width, int height) { this.width = width; this.height = height; } public Rectangle(int width, int height, int depth) { this(width, height); // Calls 2-param constructor this.depth = depth; } }; (3) Pass current object to a method: class EventSource { public void register(EventListener listener) { listener.setSource(this); // Pass this object } }; (4) Return current object (method chaining/fluent API): class QueryBuilder { private String table, condition, orderBy; public QueryBuilder from(String table) { this.table = table; return this; // Return this for chaining } public QueryBuilder where(String condition) { this.condition = condition; return this; } public QueryBuilder orderBy(String field) { this.orderBy = field; return this; } } // Usage: QueryBuilder qb = new QueryBuilder() .from("users") .where("active = true") .orderBy("name");. In Python, the equivalent is self (explicit first parameter). In JavaScript, this depends on how the function is called (more complex).