A lambda expression in Java is a short block of code that takes in parameters and returns a value. It is similar to a method, but it does not need a name and can be implemented right in the body of a method. Lambda expressions were added in Java 8 and provide a clear and concise way to represent one method interface using an expression. They are used to provide the implementation of an interface that has a functional interface, which is an interface with only one abstract method.

The syntax of a lambda expression consists of three components: argument-list, arrow-token, and body. The argument-list can be empty or non-empty, and the body contains expressions and statements for the lambda expression. The arrow-token is used to link the argument-list and the body of the expression.

Lambda expressions can be used to iterate, filter, and extract data from a collection. They are very useful in the collection library and can save a lot of code. Lambda expressions can be stored in variables if the variables type is an interface that has only one method. The lambda expression should have the same number of parameters and the same return type as that method.

Lambda expressions are just like functions and they accept parameters just like functions. The body of a lambda expression can contain zero, one, or more statements. Expressions are limited, and they have to immediately return a value, and they cannot contain variables, assignments, or statements such as if or for.