A block of code that can be represented as an object (or first-class data structure) and placed seamlessly into the flow of code by allowing it to reference its lexical scope.
aka lambda, block, or anonymous function
You have a collection of objects and want to filter them in various ways. Writing a method for each filter leads to duplication in the setup and processing of the filter.
By using a Closure, you can factor the setup and processing of the filter and pass in an arbitrary block of code for each filter condition.
For more details see chapter 37 of the DSL book