A dot placed under a symbol to indicate a dummy variable, e.g., 
(Comtet 1974, p. 32). This
notation, however, is not very common.
Underdot
See also
Dummy VariableExplore with Wolfram|Alpha
References
Comtet, L. Advanced Combinatorics: The Art of Finite and Infinite Expansions, rev. enl. ed. Dordrecht, Netherlands: Reidel, p. 32, 1974.Referenced on Wolfram|Alpha
UnderdotCite this as:
Weisstein, Eric W. "Underdot." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Underdot.html