Undefined
Something undefined has not been defined yet. The definition may vary from structure to structure: a variable is created upon its first writing, while a class needs a declaration.
There are many error messages, that are raised when attempting to use an undefined structure. The reasons include typos, missing compiled or custom code, order of execution.
<?php
// class Elephpant is defined, or declared
class Elephpant {}
// variable $trunk is undefined
echo $trunk;
?>