The act (and art) of using a QueuingModel to describe a system (proposed or actual). So why do you need a queuing model anyway? When you have just one user of a system, response times are determined by actual service times. When you have lots of users, response times start getting dominated by queues (waiting times for service) rather than service times. A QueuingModel allows you to analyze this effect.
-- RamkumarRamaswamy

----

With a lot of experience in a particular design space, it becomes possible to model performance by just relating frequently occurring design elements. For example, ''Improving .NET Application Performance and Scalability'' describes performance modeling this way in chapter 2. See http://msdn.microsoft.com/library/en-us/dnpag/html/scalenetchapt02.asp