So I looked up valency on chem wikis, wikipedia and a bunch of textbooks. But everywhere I looked the definitions turned out to be different. The IUPAC defines it as the number of univalent atoms or groups that can combine with or be replaced with one atom of the element under consideration. I find this confusing given that we're using the concept of valency (in the word univalent) in its definition. How is that a legitimate defintion? Can something be defined by itself? I would appreciate a precise and universal defintion of valency. And if it's impossible to define it in that manner, I'd be content with a short analysis of how to describe it conpletely, in a few sentences.