Master = Doctor = Teacher

I read this today and I am so inspired. I never knew this. That the Ph.D was supposed to mean an expert teacher. :) Yes, in the past having a Ph.D meant you were a good teacher. I hope this inspires me. I've always wanted to be a teacher. And now... I am indeed training to be one, because I do have an oral defense eventually coming up, where I get to "lecture" my examiners and "discuss" my work with them. Thinking of it this way takes the sting out of what traditionally would have caused a lot of fear in Ph.D students.

To this day, the names we give our university degrees and the rituals we attach to them reflect those fundamental connections between knowing and teaching. For example, the highest degrees awarded in any university are those of "master" or "doctor," which were traditionally interchangeable. Both words have the same definition; they mean "teacher." "Doctor" or "dottore" means teacher; it has the same root as "doctrine," or teaching. Master, as in school master, also means teacher. Thus, the highest university degree enabled its recipient to be called a teacher. (p. 196)

Consider the full current form of the oral exam. First, the candidate presents a brief oral exposition of the thesis. He then defends the thesis in dialogue with the examiners. These parallel the two modes of teaching: the lecture and the disputation. The oral examination is the ultimate test of subject matter expertise; it examines the candidate's ability to teach the subject by employing the dual method of lecture and discussion. (p. 197) 

Reference
Shulman, L. S. (1986). Those who understand: knowledge growth in teaching. In S. Wilson (ed.) (2004). The wisdom of practice: essays on teaching, learning and learning to teach (pp. 189-215) USA: Jossey Bass.