(PDS) A data set on an IBMmainframe that contains mEMbers, each of which acts like a separate data set. Partitioned data sets are more space-efficient than individual data sets, because they can put more than one data set on a track. They are also used to hold libraries, with one function per mEMber. The syntax for a mEMber is NAME.OF.PDS(MEMBER) although some systEMs (such as Phoenix) could use NAME.OF.PDS:MEMBER Original PDSes were of fixed size, and needed frequent compression to recover space after deleting or changing mEMbers. Newer PDS/E Extended PDSes do not have this problEM. (2003-12-05)