What is, or could be the formal and/or officially established definion of a "stage"? Many of us use this term in daily business quite intuitively: "build stage", "test stage" and used long before the rise of DevOps. 

But is there a formal and/or well-established definition of this term? 

Additional question: Who and where used it for first time?
Often in technology we co-opt English words and give them alternate meanings. However, in this case it's really just the standard definition:


  A level, degree, or period of time in the course of a process