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In linear algebra, a 'reflection' is a linear transformation that squares to the identity. In addition to reflections across hyperplanes, the class of general reflections includes point reflections, reflections across subspaces of intermediate dimension, and non-orthogonal reflections.
Any reflection is diagonalizable, with eigenvalues of –1 and 1. The fixed point set of a general reflection (i.e., the eigenspace corresponding to the eigenvalue 1) is known as its 'mirror'.