SpellingOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for notebooks that specifies settings for spell checking.

SpheroidalJoiningFactor[n, m, \[Gamma]] gives the spheroidal joining factor with degree n and order m.

SpheroidalRadialFactor[n, m, c] gives the spheroidal radial factor with degree n and order m.

SquaresR[d, n] gives the number of ways r_d (n) to represent the integer n as a sum of d squares.

StepMonitor is an option for iterative numerical computation functions that gives an expression to evaluate whenever a step is taken by the numerical method used.

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], ....

Switch[expr, form_1, value_1, form_2, value_2, ...] evaluates expr, then compares it with each of the form_i in turn, evaluating and returning the value_i corresponding to ...

SynchronousUpdating is an option for Manipulate, Dynamic, and related functions that specifies whether or not to evaluate their contents synchronously.

SyntaxForm is an option for InterpretationBox and TagBox objects that specifies the precedence level to use when the InterpretationBox or TagBox is considered as an operator.

SystemInformation[] gives detailed information about the Mathematica system being run. SystemInformation["comp"] gives a list of rules with information about the component " ...