Most computational phonologists (and morphologists) agree that the surface properties of language can be modelled by finite automata (FAs), finite state machines (FSMs), i.e. finite state transducers (FSTs).
Definition (Hopcroft & Ullman):
A finite automaton (FA) consists of a finite set of states and a set of transitions frojm state to state that occur on input symbols chosen from an alphabet 
. For each input symbol there is exactly one transition out of each state (possibly back to the state itself). One state, usually denoted 
, is the initial state, in which the automaton starts. Some states aredesignated as final or accepting states.
Formal model for automata:
where:
Q is a finite set of states
is a finite input alphabet
is the initial state
is the set of final states
is the transition function 
LP: Single FSMs
QLP: Parallel FSMs