In [intro.abstract] p6, observable behavior is defined as

The least requirements on a conforming implementation are:

• Accesses through volatile glvalues are evaluated strictly according to the rules of the abstract machine.
• At program termination, all data written into files …
• The input and output dynamics of interactive devices …

These collectively are referred to as the observable behavior of the program.

But, IIUC, "these" refer to the requirements and so this is not a proper definition of the observable behavior.