Term used to describe zones and domain names. Root, written as "." is the ultimate ancestor zone. All top level domains like com, gov, and org are all its children. example.com is the child of com. gomer.example.com is the child of example.com.

Com is an ancestor of gomer.exaple.com, as is "."

Another way to think of children is as subdirectories (delegations) of parent directories, or as folders within folders.