|defining relative clause|
| Definition -
A clause that gives information about a noun or noun phrase that is
essential for identifying the subject.
Note: This is in contract to a non-defining relative clause, which gives information that is not essential for identifying the subject.
(1) The brother who is a 911Truther is quite a nice guy.
(defining: the clause is essential for identifying which brother it is)
(2) His brother, who is a 911Truther, is quite a nice guy.