Custom
A feature is called custom when it is not part of the standard installation of PHP, any PHP extension or a publish library. For example, the function trumpet() is generally a custom function.
Any declaration object may be custom: constants, variables, classes, interfaces, enums, traits.
A custom asset is usually the last to be defined, and, as such, has to use a distinct name. For example, declaring a function called wp_header is possible as long as the code is not part of Wordpress, which does declare it as a common set of features.
<?php
// This is a custom function
function trumpet() {}
?>