NULL, nil, null, undef

nil.md

C/C++

void* a = NULL; // not sure about all compiler

C++11

void* a = nullptr;

Objective C

id a = nil;

C#

object a = null;

Ruby

a = nil

Perl

my $a = undef;

Python

a = None

Javascript

var a = undefined;

VB

Dim a as Integer
a = Nothing

Java

Integer a = null;

Shell

a=

Lisp

(setq a nil)