Undefined variable

An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been previously declared by that code.
In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails.
Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. Perl's "codice_1" or Visual Basic's "codice_2").
Examples.
The following provides some examples of how various programming language implementations respond to undefined variables. Each code snippet is followed by an error message (if any).
CLISP.
 *** - EVAL: variable X has no value
C.
int main() {
 int y = x;
 return 0;
 foo.c: In function `main':
 foo.c:2: error: `x' undeclared (first use in this function)
 foo.c:2: error: (Each undeclared identifier is reported only once
 foo.c:2: error: for each function it appears in.)
JavaScript.
y = x
 Error: x is not defined
 Source File: file:///c:/temp/foo.js
Lua.
y = x
print(y)
 nil
ML (Standard ML of New Jersey).
val y = x;
 stdIn:1.9 Error: unbound variable or constructor: x
MUMPS.
 Set Y=X
 <UNDEF>
OCaml.
let y = x;;
 Unbound value x
Perl.
my $y = ($x // 0) + 1; # defined-or operator
 (no error)
PHP 5.
$y = $x;
 (no error)
$y="";
$x="";
error_reporting(E_ALL);
$y = $x;
 PHP Notice: Undefined variable: x in foo.php on line 3
Python 2.4.
»> x = y
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined
REXX.
signal on novalue
y = x
 +++ Error 30 in line 2: Label not found
Ruby.
irb(main):001:0> y = x
NameError: undefined local variable or method `x' for main:Object
 from (irb):1
Tcl.
% set y $x
can't read "x": no such variable
VBScript.
Dim y
y = x
 (no error)
Option Explicit
Dim y
y = x
 (3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'