deficiency, deficiency number, of an operator
The dimension $\dim D_\lambda$ of the deficiency subspace of the linear operator. Occasionally, in a wider sense, the deficiency of a linear manifold in a Hilbert space is defined to be the dimension of the orthogonal complement of this manifold.
Defect. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Defect&oldid=31947