Nondeterminism means that the path of execution isn't fully determined by the specification of the computation, so the same input can produce different outcomes, while deterministic execution is guaranteed to be the same, given the same input. Related terms to "nondeterministic" are "probabilistic" and "stochastic".

The source of nondeterminism must be outside of the specification; for example, a common source of nondeterminism is a random number generator. Languages like C allow for "unspecified behavior" where the compiler implementers can apply different optimizations to the execution model, so the same source code (specification) could compile to different executables. Concurrency is a natural source of nondeterminism since the order and timings of independent processes can vary; for example, a server with multiple independent clients can receive requests at random times.