Possible Duplicate:
What is it to be normal?
I've learned in algebra class that
"normal" means a linear operator is commutative with its adjoint;
also we say that $H$ is a normal subgroup of $G$ if $gH=Hg$ for all $g\in G$.
It seems that the word "normal" mostly concerns with the commutative properties, I wonder what other meanings does it have, and why we name it like this~
Any comment will be a help~