An accent symbol placing a wide circumflex over multiple characters, commonly used for estimators in statistics or operators in linear algebra.
Examples
Angle measure in geometry
\widehat{AB} = 60°
Arc length notation
\text{length of } \widehat{PQ} = 2\pi r
Fourier transform notation
\widehat{f}(\xi) = \int_{-\infty}^{\infty} f(x)e^{-2\pi ix\xi}\,dx