TabSpacings is an option for character selections that specifies the number of spaces in ems that the cursor advances when the Tab key is pressed.
ThompsonGroupTh[] represents the sporadic simple Thompson group Th.
UnderBar[expr] displays with a bar under expr.
Underlined represents an underlined font.
UndirectedGraph[g] gives an undirected graph from the directed graph g.
VertexSize is an option and property for Graph and related functions that specifies the size used for vertices.
White
(Built-in Mathematica Symbol) White represents the color white in graphics or style specifications.
Whitespace represents a sequence of whitespace characters in StringExpression.
WordBoundary represents a boundary between words for purposes of matching in StringExpression.
WordSearch is an option for Find and FindList that specifies whether the text searched for must appear as a word.