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 valid as an expression—it evaluates to itself. But its main use is not to be evaluated as an expression, but to be called as a function.
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 Function Names). Before going into these details, the following subsections describe the components of a lambda expression and what they do.