In hiërarchische systemen wordt gesproken over parent processes en child processes. De child processes zijn daarbij ondergeschikt aan de parent processes. Ze liggen dus een niveau lager.

wait suspends the calling process until one of the immediate children
terminate, or until a child that is being traced stops because it has hit
an event of interest. The wait will return prematurely if a signal is
received. If all child processes stopped or terminated prior to the call
on wait, return is immediate.

If the call is successful, the process ID of a child is returned.

wait3 is BSD's extension of wait. It provides an alternate interface for
programs that must not block when collecting the status of child
processes.

waitpid is POSIX's extension of wait. The pid argument specifies a set
of child processes for which status is requested. waitpid only returns
the status of a child process from this set.
http://www.mcsr.olemiss.edu/cgi-bin/man-cgi?wait 2

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Note added at 31 mins (2005-02-09 11:14:20 GMT)
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tree
n. A data structure containing zero or more nodes that are linked together in a hierarchical fashion. If there are any nodes, one node is the root; each node except the root is the child of one and only one other node; and each node has zero or more nodes as children. See also child (definition 2), graph, leaf, node (definition 3), parent/child (definition 2), root.
http://support.microsoft.com/?scid=http://support.microsoft....