I would very much like to know

When did identity become such a central concept? Specifically, I would like to know when the word came into common usage in sociological texts, but also when it achieved popular usage as well. The text than I'm reading from the '60s doesn't make use of the word, but I'm not sure if that is because it wasn't used then, or if the author didn't see it as useful to his text. The more recent book I'm reading uses the term identity regularly. The other question would be, was there discussion of a markedly similar to that which we now call identity, or is the concept of identity relatively new? And which contemporary definition of identity does a possible identity-like term match?