In functional analysis, the compression of a linear operator T on a Hilbert space to a subspace K is the operator P K T | K : K → K {\displaystyle P_{K}T\vert _{K}:K\rightarrow K},where P K : H → K {\displaystyle P_{K}:H\rightarrow K} is the orthogonal projection onto K. This is a natural way to obtain an operator on K from an operator on the whole Hilbert space. Wikipedia