dissertation [dissertations](formal exposition of a subject) noun [UK: ˌdɪ.sə.ˈteɪʃ.n̩] [US: ˌdɪ.sər.ˈteɪʃ.n̩]
thesis [theses](lengthy essay written to establish the validity of a thesis, especially one submitted as a requirement for a university degree) noun [UK: ˈθiːs.ɪs] [US: ˈθiːs.ɪs]