In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Wikipedia
Use lambda functions when an anonymous function is required for a short period of time.
has no name but takes in some argument or arguments, and returns a value immediately.

Bound to variable but not identifier, no parentheses or return keyword

Typically lambdas are used to encapsulate a few lines of code that are passed to algorithms or asynchronous functions.