Atomic semantics are defined formally in Lamport's "On Interprocess Communication" Distributed Computing 1, 2 (1986), 77-101. (Also appeared as SRC Research Report 8).
Atomic semantics are defined for a variable with a single writer but multiple readers. These semantics are very strong: they guarantee that the read and write operations to the variable behave exactly as if they happened instantaneously in some point in time which is within the actual time where the operation took place.
see also: Regular semantics and Safe semantics