|Definition - A clause that is connected to one or more clauses of equal status.|
Example - I yelled as loud as I could, but she couldn't hear me.
In the above the two coordinate clauses are
(1) I yelled as loud as I could and
(2) she couldn't hear me
They are connected by the coordinating conjunction but.
Oxford English Dictionary - The term's first citation is from 1871:
"A Coordinate Clause is not governed in its construction by the Principal Sentence."
(Public Sch. Lat. Gram. §151 )