subtype

A subtype is any of the child types. It is the classes that extend the target class, with the extends keyword; it also covers all the classes that are a subtypes of that child class.

The opposite is supertype.

<?php

// Aparent is a supertype of AChild
class Aparent extends Agrandparent {}

class Achild extends Aparent{}

// Agrandchild is a subtype of AChild
class Agrandchild extends Achild {}

// These are not subtype of AChild
class ABrother extends Aparent{}

class ASister extends Aparent{}

?>

Related : supertype