A data set is a collection of related, discrete items of related data that may be accessed individually or in combination or managed as a whole entity.
A data set is organized into some type of data structure. In a database, for example, a data set might contain a collection of business data (names, salaries, contact information, sales figures, and so forth). The database itself can be considered a data set, as can bodies of data within it related to a particular type of information, such as sales data for a particular corporate department.
The term data set originated with IBM, where its meaning was similar to that of file. In an IBM mainframe operating system, a data set s a named collection of data that contains individual data units organized (formatted) in a specific, IBM-prescribed way and accessed by a specific access method based on the data set organization. Types of data set organization include sequential, relative sequential, indexed sequential, and partitioned. Access methods include the Virtual Sequential Access Method (VSAM) and the Indexed Sequential Access Method (ISAM).
A data set is also an older and now deprecated term for modem.