A lambda expression is a function object written in Lisp. Here is an example:
(lambda (x) "Return the hyperbolic cosine of X." (* 0.5 (+ (exp x) (exp (- x)))))
In Emacs Lisp, such a list is a valid expression which evaluates to a function object.
A lambda expression, by itself, has no name; it is an anonymous function. Although lambda expressions can be used this way (see Anonymous Functions), they are more commonly associated with symbols to make named functions (see Naming a Function). Before going into these details, the following subsections describe the components of a lambda expression and what they do.