Locked is an attribute that, once assigned, prevents modification of any attributes of a symbol.

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.

Medium is a style or option setting that specifies that objects should be medium sized.

MenuCommandKey is an option for cells that specifies the keyboard shortcut to be associated with a style listed in the Format \[FilledRightTriangle] Style submenu.

MenuPacket[integer, string] is a MathLink packet indicating a menu request with title string.

MessagePacket[symbol, string] is a MathLink packet containing a Mathematica message identifier of the form symbol::string.

-x is the arithmetic negation of x.

Modal is an option to functions such as CreateDialog that specifies whether the dialog that is created should be modal to the Mathematica front end.

MorphologicalBranchPoints[image] gives a version of a binary image image in which white pixels represent the morphological branch points.

NameQ["string"] yields True if there are any symbols whose names match the string pattern given, and yields False otherwise.