Groovy: what's the purpose of “def” in “def x = 0”?

def x = 0
def y = 5

while ( y-- > 0 ) {
    println "" + x + " " + y
    x++
}

assert x == 5

x = 1
assert x == 1
assert this.binding.getVariable("x") == 1

def y = 2

assert y == 2

try {
    this.binding.getVariable("y")
} catch (groovy.lang.MissingPropertyException e) {
    println "error caught"
}

x = 1
 def y = 2


public bar() {
    assert x == 1

    try {
        assert y == 2
    } catch (groovy.lang.MissingPropertyException e) {
        println "error caught"
    }
}

bar()

// Groovy imports java.io.* and java.util.* automatically
// but not java.nio.*

import java.nio.channels.*

class Foo {
    public void bar() {
        FileChannel channel = new FileInputStream('Test.groovy').getChannel()
    	println channel.toString()
    }
}

new Foo().bar()

// Groovy imports java.io.* and java.util.* automatically
// but not java.nio.*
class Foo {
    public void bar() {
        def channel = new FileInputStream('Test.groovy').getChannel()
    	println channel.toString()
    }
}

new Foo().bar()