InverseFunctions is an option for Solve and related functions that specifies whether inverse functions should be used.
JoinForm[type] is a graphics directive that specifies what type of joins should be used to connect segments of lines, tubes, edges, and related primitives.
LibraryUnload[lib] unloads all functions that have been loaded from a Wolfram Library, then it unloads the library.
MakeExpression[boxes, form] is the low-level function used in Mathematica sessions to construct expressions from boxes.
NHoldRest is an attribute which specifies that all but the first argument to a function should not be affected by N.
NumberFieldIntegralBasis[a] gives an integral basis for the field \[DoubleStruckCapitalQ][a] generated by the algebraic number a.
ObservableModelQ[ss] gives True if the StateSpaceModel object ss is observable, and False otherwise.
Polygon
(Built-in Mathematica Symbol) Polygon[{pt_1, pt_2, ...}] is a graphics primitive that represents a filled polygon. Polygon[{{pt_11, pt_12, ...}, {pt_21, ...}, ...}] represents a collection of polygons.
Regularization is an option for Sum and Product that specifies what type of regularization to use.
RightCosetRepresentative[group, g] returns the smallest element in the right coset of products of the elements of group by g.