This is the possibly infinite dimensional version of a Hermitian matrix, since linear operators are the possibly infinite dimensional version of matrices.
There's a catch though: now we don't have explicit matrix indices here however in general, the generalized definition is shown at: en.wikipedia.org/w/index.php?title=Hermitian_adjoint&oldid=1032475701#Definition_for_bounded_operators_between_Hilbert_spaces
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The Spectral Theorem is a fundamental result in linear algebra and functional analysis that pertains to the diagonalization of certain types of matrices and operators. It provides a relationship between a linear operator or matrix and its eigenvalues and eigenvectors.