The term data set or dataset is used to refer to files on an IBM mainframe computer, typically stored on DASD or magnetic tape. They are record-oriented files.
Unlike files on UNIX systems, they are not unstructured streams of bytes but rather are organized in various logical record and block structures determined by the DSORG (data set organisation) parameter of the JCL that was used to allocate them.
For example, a PDS or Partitioned Data Set is a dataset containining multiple members, each of which holds a separate sub-data set, similar to a directory in other types of file system.
Since MVS/XA there is also the Partitioned DATA set Extended (PDSE). Here the data in 4K become - blocks administered and free the place become when the deletion Members can be used without Compress immediately. The directory size is completely variable. The introduction of PDSE runs very slowly, there until recently integrity problems existed.