In computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, concurrency is a property of systems in which several computation

Computation

Computation is defined as any type of calculation. Also defined as use of computer technology in Information processing.Computation is a process following a well-defined model understood and expressed in an algorithm, protocol, network topology, etc...

s are executing  simultaneously, and potentially interacting with each other. The computations may be executing on multiple cores in the same chip, preemptively time-shared

Preemption (computing)

In computing, preemption is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time. Such a change is known as a context switch...

 threads

Thread (computer science)

In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...

 on the same processor, or executed on physically separated processors.
A number of mathematical models have been developed for general concurrent computation including Petri nets, process calculi, the Parallel Random Access Machine

Parallel Random Access Machine

In computer science, Parallel Random Access Machine is a shared memory abstract machine. As its name indicates, the PRAM was intended as the parallel computing analogy to the random access machine...

 model, the Actor model

Actor model

In computer science, the Actor model is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and...

 and the Reo Coordination Language

Reo Coordination Language

In computer science, Reo is a channel-based exogenous coordination language.- Overview :In the context of distributed application, correction and validity of single processes does not imply the correctness of the whole system. In such systems, the communication between the processes is of a high...

.
Because computations in a concurrent system can interact with each other while they are executing, the number of possible execution paths in the system can be extremely large, and the resulting outcome can be indeterminate.