What is the standard name for the function which inflates a string by duplicating each of its characters? Given a string $s$ over some alphabet, I'd like to use the proper nomenclature/notation for the operation/function $f$ which inflates $s$ by independently duplicating each of its characters.

For instance:
$$ f(abc) = aabbcc$$
or, more generally, a second argument could be provided to specify the number of copies:
$$ f(xy,4) = xxxxyyyy$$

This should be embarassingly standard, so don't hesitate to shoot me in your response! 
(on the other hand, it is always difficult to search for something you cannot name, but I wouldn't accept such a lame excuse from my students, so...)
As pointed out by Marcus Ritt, the correct name for this operation seems to be stutter. 

As far as I could determine, it has mostly been used in the field of concurrency theory, where I could trace it back at least to Lamport's 1983 seminal paper on temporal logic.