You may be able to reuse some terminology from software world here.
There is a standard software design pattern called Observer and its terminology may help. Quoting wikipedia:
The observer pattern is a software design pattern in which an object,
called the subject, maintains a list of its dependents, called
observers, and notifies them automatically of any state changes,
usually by calling one of their methods.
Also there is a Observable interface which may be implemented by observable objects. Quoting its description:
An observable object can have one or more observers.
[I know subject was a previous answer but I think this was too long as a comment.]