Little by little, and above all in the modern era, the term became synonymous with an argumentative piece of writing, serving and asserting an idea.
In effect, the organisation of university studies made it necessary to have a more precise definition, which create the level of "maîtrise".
The pursuit of apprenticeships in research work involved students in editing a "second mémoire", at Master 2 (formerly DEA) level, often as a feasibility study for a full thesis.
For a Master 2 in social sciences or literature, a "mémoire" comprises a description of the sources which would allow future work on a thesis, the necessary bibliography, an exposé of the problems involved as defined by the student, and one or two attempts at edited-down chapters, showing that the problems, sources and bibliography could lead to a real work of original research.
On the other hand, one rarely speaks of a "mémoire de thèse", as this is too voluminous, and its usage is only to qualify that the thesis is all the student's own work.