LetterCharacter represents a letter character in StringExpression.
LibraryFunctionError[name, code] represents an error returned from a LibraryFunction.
MakeBoxes[expr, form] is the low-level function used in Mathematica sessions to convert expressions into boxes. MakeBoxes[expr] is the function to convert expr to ...
MatchLocalNames is an option for Trace and related functions that specifies whether symbols such as x should match symbols with local names of the form x$nnn.
NotebookAutoSave is a notebook option which specifies whether the notebook should automatically be saved after each piece of output generated by evaluation in it.
NullRecords is an option for Read and related functions which specifies whether null records should be taken to exist between repeated record separators.
NumberMultiplier is an option for NumberForm and related functions which gives the string to use as a multiplication sign in scientific notation.
NumberPoint is an option for NumberForm and related functions which gives the string to use as a decimal point.
Order
(Built-in Mathematica Symbol) Order[expr_1, expr_2] gives 1 if expr_1 is before expr_2 in canonical order, and -1 if expr_1 is after expr_2 in canonical order. It gives 0 if expr_1 is identical to expr_2.
OverBar
(Built-in Mathematica Symbol) OverBar[expr] displays with a bar over expr.