👉 A self-adjoint operator in mathematics is an operator that has a corresponding unitary operator as its adjoint. In other words, it's defined by its own eigenvalues and eigenvectors. Selfadjoint operators are important because they can be used to represent linear transformations on Hilbert spaces, which are infinite-dimensional vector spaces with a countable number of dimensions. They also have many applications in quantum mechanics, particularly in the study of quantum superposition and entanglement.