A diafix of a string is a substring (). It is therefore not a first (prefix) or last (suffix) part of a string, but rather is a "center" part.
Diafix
See also
Prefix, String, SuffixThis entry contributed by Peter van der Helm
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References
van der Helm, P. A. "Transparallel Processing by Hyperstrings." Proc. Nat. Acad. Sci. USA 101, 10862-10867, 2004.Referenced on Wolfram|Alpha
DiafixCite this as:
van der Helm, Peter. "Diafix." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/Diafix.html