1 A lengthy piece of academic writing presenting an argument based on research, and presented for assessment towards an academic award at postgraduate level, such as a master's degree or a doctorate. A thesis at this level is usually expected to contribute something new to existing knowledge within its field.
2 An original idea or contribution to knowledge, for which a carefully argued case is presented, usually, but not always, in the form of sense 1 above.
Thus, the term may refer to the written document or to the argument contained within the text. See also dissertation.