What does the backslash mean in math?

Answer 1

The backslash, \ , usually refers to the relative complement of two sets.

Given two sets P and Q, Q \ P is the set of elements in Q that aren't in P.

For example, {Cy Young, CC Sabathia, David Price, Felix Hernandez} \ {CC Sabathia, David Price} = {Cy Young, Felix Hernandez}.