An accent symbol placing a wide tilde over multiple characters, commonly used for equivalence relations or function transforms.
Examples
Denoting a transformed or modified variable
\widetilde{x} = f(x)
Representing a space with special properties
\widetilde{M} = M/\sim \text{ (quotient space)}
Indicating a lifted or extended function
\widetilde{f}: \widetilde{X} \rightarrow \widetilde{Y}