Before we discuss queues, we must introduce a little vocabulary. When we describe queueing systems, we refer to the queue, which is the waiting work, and the server, which is the resource performing the work. The pattern with which work arrives, which is usually unpredictable, is known as the arrival process. The time it takes the server to accomplish the work may also be unpredictable. This is known as the service process. We handle waiting work in a certain sequence, such as first-in-first-out (FIFO). This sequence is known as the queueing discipline.
Donald G Reinertsen in The Principles of Product Development Flow