A scriptable object is an object that records the operations done to it and it can store them as a "script" which can be replayed.
For example, see: Application Scripting Framework
Now, if Alistair didn't know what he asked and really meant "subscriptable" objects (as edited by others), then (as mipadi also answered) this is the correct one:
A subscriptable object is any object that implements the __getitem__
special method (think lists, dictionaries).