Is "zed" a valid name for $\emptyset$? I've always known that $\emptyset$ is called an _empty set_ or _null_ , until recently, when I heard someone calling it _zed_. I looked it everywhere but couldn't find this naming. Is " _zed_ " a valid name for the $\emptyset$ symbol?

Personally, I have never heard it called that (of course, it is always possible that some in subfield of mathematics I'm not familiar with, it is an accepted name). However, non-Americans will usually call $\mathbb{Z}$ (the integers) "zed", perhaps you misunderstood?