The backslash symbol is used to denote a set difference, quotient group, or integer division.
Backslash
See also
Integer Division, Quotient Group, Set Difference, Solidus, VirguleExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Backslash." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Backslash.html