Introducción

En palabras de la Wikipedia:

In computer science, a closure is a first-class function with free variables that are bound in the lexical environment. Such a function is said to be "closed over" its free variables. A closure is defined within the scope of its free variables, and the extent of those variables is at least as long as the lifetime of the closure itself.

Un ejemplo:

generaciondecodigos@nereida:~/src/groovy/closures$ cat -n closure.groovy
     1  #!/usr/bin/env groovy
     2
     3  def p(x) {
     4    return { y -> x + y}
     5  }
     6
     7  p2 = p(2)
     8
     9  println p2(5)
    10  println p2(9)
    11
    12  println p2(4.5)
    13
    14  ph = p("hola ")
    15  println ph("mundo")
    16

Al ejecutar este programa obtenemos la salida:

generaciondecodigos@nereida:~/src/groovy/closures$ ./closure.groovy
7
11
6.5
hola mundo