dis·ser·ta·tion (dĭs'ər-tā'shən) n. A lengthy, formal treatise, especially one written by a candidate for the doctoral degree at a university; a thesis.
1611, from L. dissertationem (nom. dissertatio) "discourse," from dissertare "debate, argue," frequentative of disserere "discuss, examine," from dis- "apart" + serere "to arrange words" (see series). Sense of "formal, written treatise" is 1651.