Define the thread root

content/client-server-api/modules/threading.md

@@ -12,7 +12,7 @@ as by providing some context to what is going on in the thread but keeping the f
 history behind a disclosure.
 
 Threads are established using a `rel_type` of `m.thread` and reference the
-*thread root*. It is not possible to create a thread from an event which itself
+*thread root* (the main timeline event to which the thread events refer). It is not possible to create a thread from an event which itself
 is the child of an event relationship (i.e., one with an `m.relates_to`
 property). It is therefore also not possible to nest threads. All events in a
 thread reference the thread root instead of the most recent message, unlike rich