dissertation
n. US //ˌdɪsɝˈteɪʃən// UK //dɪsətˈeɪʃən// dis·ser·ta·tion
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1 lengthy speech or writing (n.) C2 Proficiency Formal Literarya long, serious speech or piece of writing about a subject.
a lengthy and systematic discourse or treatise on a particular topic, not necessarily for academic credit.
ExampleHe gave us a long dissertation on why the old laws were better.
ExampleThe professor launched into a spontaneous dissertation on the evolution of gothic architecture, ignoring the original topic of the seminar.
From Latin dissertātiō, from dissertō.