I was looking that up and found that in UNIX v6 source code, the field name in kernel source code is u_cdir, which stands for "current directory of (user) process":

https://github.com/yisooan/unix-v6/blob/master/sys/user.h

I started writing this comment:

A variable holding this most often is called fwd, though (current working directory)

Then I did some thinking and research and there is the PWD environment variables. Might be interesting if that predates the pwd command or not.