Why is the reduced norm called reduced?

Question

The reduced norm of an element $a\in A$ of a finite-dimensional algebra over a field $K$ is defined to be $$ N_{rd,K}(a)=\det(M_a), $$ where $M_a$ is the $K$-linear map $x\mapsto ax$ on $A$. My question is: why is it called ``reduced''? Is there such a thing as an unreduced norm?