$RecursionLimit gives the current limit on the number of levels of recursion that Mathematica can use.

ReturnPacket[expr] is a MathLink packet that contains the expression expr, the result of an EvaluatePacket evaluation.

$Pre is a global variable whose value, if set, is applied to every input expression.

ReturnTextPacket[string] is a MathLink packet containing string, the result of an EnterTextPacket evaluation.

EnterTextPacket[string] is a MathLink packet that requests the parsing and evaluation of string as an expression.

Refine[expr, assum] gives the form of expr that would be obtained if symbols in it were replaced by explicit numerical expressions satisfying the assumptions assum. ...

CellEpilog is an option for Cell which gives an expression to evaluate after each ordinary evaluation of the contents of the cell.

TraceOriginal is an option for Trace and related functions which specifies whether to test the form of each expression before its head and arguments are evaluated.

HoldRest is an attribute which specifies that all but the first argument to a function are to be maintained in an unevaluated form.

HoldFirst is an attribute which specifies that the first argument to a function is to be maintained in an unevaluated form.