Subfactorial[n] gives the number of permutations of n objects that leave no object fixed.
Subgraph[g, {v_1, v_2, ...}] gives the subgraph of the graph g generated by the vertices v_i.Subgraph[g, {e_1, e_2, ...}] gives the subgraph generated by the edges ...
SubMinus[expr] displays as expr_-.
SubPlus
(Built-in Mathematica Symbol) SubPlus[expr] displays as expr_+.
Subresultants[poly_1, poly_2, var] generates a list of the principal subresultant coefficients of the polynomials poly_1 and poly_2 with respect to the variable var.
SubscriptBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for SubscriptBox objects.
Subscript[x, y] is an object that formats as x_y. Subscript[x, y_1, y_2, ...] formats as x Subscript[y, 1], Subscript[y, 2], ....
Subsets
(Built-in Mathematica Symbol) Subsets[list] gives a list of all possible subsets of list. Subsets[list, n] gives all subsets containing at most n elements. Subsets[list, {n}] gives all subsets containing ...
SubsuperscriptBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for SubsuperscriptBox objects.
Subsuperscript[x, y, z] is an object that formats as x_y^z.