ManhattanDistance[u, v] gives the Manhattan or "city block" distance between vectors u and v.
MathieuS[a, q, z] gives the odd Mathieu function with characteristic value a and parameter q.
MaxIterations is an option that specifies the maximum number of iterations that should be tried in various built-in functions and algorithms.
MeshShading is an option for plotting functions that gives lists of colors to use for regions between mesh divisions.
NumberFieldIntegralBasis[a] gives an integral basis for the field \[DoubleStruckCapitalQ][a] generated by the algebraic number a.
Opener
(Built-in Mathematica Symbol) Opener[x] represents an opener with setting x, displayed as OpenerBox[True] when x is True and OpenerBox[False] when x is False. Opener[Dynamic[x]] takes the setting to be ...
OpenWrite["file"] opens a file to write output to it, and returns an OutputStream object. OpenWrite[] opens a new file in the default area for temporary files on your ...
Pi
(Built-in Mathematica Symbol) Pi is \[Pi], with numerical value \[TildeEqual] 3.14159.
RegionFunction is an option for plotting functions that specifies the region to include in the plot drawn.
SimpleGraph[g] gives the underlying simple graph from the graph g.