The term data set or dataset is used to refer to files on an IBM mainframe computer, typically stored on DASD or magnetic tape.
Unlike files on UNIX systems, they are not unstructured streams of bytes, but rather are organized in various block structures determined by the DSORG (data set organisation) parameter of the JCL which was used allocated 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.