Unified Modeling Language Reference Manual, The, Second Edition

atomic

An action or operation whose execution must be completed as a unit; one that may not be partially executed or terminated by an external event or interrupt. Usually, atomic operations are small and simple, such as assignments and simple arithmetic or string calculations. An atomic computation occurs at a definite point in the execution sequence.

This is not a precise UML term.

See also action, do activity, run-to-completion.

Semantics

Conceptually, atomic actions that do not access shared resources simply require concurrent execution. There are many ways to map such actions onto shared processor resources as part of an implementation.

If actions access shared resources (memory, devices, or external interactions), they can hardly be considered ...