As used in this chapter
"action" means any civil action or proceeding brought in or appealed to any court which is subject to these rules;
"defendant" means any party originally named as a defendant, any party added to the record as a defendant and any plaintiff against whom a counterclaim or setoff is asserted;
"plaintiff" means any party prosecuting the action, or any defendant insofar as such defendant asserts any counterclaim or setoff against any plaintiff in the action.
231 Pa. Code r. 2301