Expression for $order(A)-rank(A)$

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I have came across a strange expression today:

the defect of the stiffness matrices K

It was used in this paper. If I understand it correctly, it's used there to describe the number of linearly dependent rows in a symmetric matrix, but I'm not able to find any definition, so I suppose there must be some more frequently used expression for this.

So, if I have a matrix $A$ of order $n$ and $rank(A) = m$, how is properly called the value $n-m$?