thesis etymology

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The term “thesis” comes from the Greek θέσις, meaning “study”, and refers to an intellectual proposition. “Dissertation” comes from the Latin dissertātiō, meaning “path”.

Originally, the words “dissertation” and “thesis” had separate meanings. When, at ancient universities, the professor had completed his lecture, there would be a disputation. During this, students could take up certain points and argue them. The position that one took during a disputation was the thesis, while the dissertation was the line of reasoning with which one supported it.
The word “thesis” comes from the Greek θέσις , meaning “position”, and refers to an intellectual proposition. “Dissertation” comes from the Latin dissertātiō, meaning “discourse”.

Источники:

http://kids.kiddle.co/Thesis
http://writingcenter.ashford.edu/writing-a-thesis