An accent mark placed above variables to indicate approximation or equivalence relations in algebra and analysis.
Examples
Denoting a transformed or modified variable
\text{Let } \tilde{x} = x - \mu \text{ be the centered variable}
Representing an estimated value
\text{The sample mean } \tilde{\mu} \text{ approximates the population mean } \mu
Indicating a corresponding element
\text{For each } a \in A \text{, there exists } \tilde{a} \in \tilde{A}