Underlined represents an underlined font.
UnderoverscriptBox[x, y, z] is the low-level box representation for x_y^z in notebook expressions.
UnderoverscriptBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for UnderoverscriptBox objects.
Underoverscript[x, y, z] is an object that formats as x_y^z.
UnderscriptBox[x, y] is the low-level box representation for UnderscriptBox[x, y] in notebook expressions.
UnderscriptBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for UnderscriptBox objects.
Underscript[x, y] is an object that formats as UnderscriptBox[x, y].
UndirectedEdge[u, v] or u \[UndirectedEdge] v represents an undirected edge of a graph with vertices u and v.
UndirectedGraph[g] gives an undirected graph from the directed graph g.
UndirectedGraphQ[g] yields True if the graph g is an undirected graph and False otherwise.