Loop and iteration are not synonyms (an iteration is one single step of a loop), but they refer to the same process, so I suggest to use a single tag. I prefer loop which has been used 243 times against the 86 times for iteration.
From a programming perspective, iterator and iteration are different. Iterators are specific objects to go through a container, available in many object-oriented languages. Definition below from C++ iterator
An iterator is any object that, pointing to some element in a range of elements (such as an array or a container), has the ability to iterate through the elements of that range using a set of operators (with at least the increment (++) and dereference (*) operators).
These iterators can be used to loop inside a container, but many loop do not require an iterator (example: while loops).
On the other hand, the term "iterator" is used by ESRI for their ModelBuilder tools (cfr list of iterators in ESRI doc):
Iterators enable batch processing and help repeat a process or set of processes on a set of inputs.
A flag "iterator" can therefore be confusing because it refers to two different concepts, but I don't expect many "pointer" related questions in GIS SE (specific "pointer" questions will probably be out of scope, in my humble opinion).
I've looked at the first 50 questions flagged with iterator. All but 4 were ArcGIS-related, and only one was really about a pointer but would have been perfectly understood with a flag like loop or iteration. Based on its use, we can say that
"iterator" is a combination of "modelbuilder" + "loop/iteration".
To sum up: merge loop and iteration.
iterator could be merged with loop, but it is more specific. If loop and modelbuilder are used together, then it is unambiguously associated with iterator. However, "loop" and "arcgis" is not sufficient, because there can be loops in arcpy, and they don't use ESRI's iterators.