An abstract machine, also called an abstract computer, is a theoretical model of a computer hardware or software system used in automata theory. Abstraction of computing processes is used in both the ComputerScience and ComputerEngineering disciplines and usually assumes discrete time paradigm.
See: http://en.wikipedia.org/wiki/Abstract_machine
See also VirtualMachine, WhatIsaModel, AnIntroductionToAutomataTheory