In the UNIX world of a single-application-per-hardware machine, the term system is often used synonymously with server. It usually refers to a hardware machine and the software that runs on it. In the mainframe environment with multiple operating systems, each running several applications on virtualized hardware, server no longer corresponds to a single hardware machine.
In this book, we use system either in a non-mainframe context or as a general term that includes the mainframe hardware and all its software, encompassing all operating systems and applications. To narrow the scope to a particular instance of an operating system, we use image. When referring to the scope of an application, we say server. Sometimes we take a user's view of an application as a single server even if its implementation involves multiple images.
The following table lists some other words that are used differently in the UNIX and mainframe worlds. Depending on the context, we use both terminologies.
For a complete list of abbreviations and special terms used in this book, see "Glossary."