This is the standard way of calling it.
The abstract is a required document to be submitted to the Thesis Examination Committee. A decent abstract answers thre questions: What, Why and How: what did you do / are you going to do?; why do you think it is/was necessary/viable?; and how did you /are you going to go about it?.
That's the standard. If you want to get "creative" and translate the text as literature rather than standard academic-bureaucratic text, then you can say:

"sustaining the topic of the Dissertation" (certainly NOT "justification"!)

Incidentally, while "thesis" is a more colloquial and more widely used version, "dissertation", too, is not only perfectly legitimate but also a rather more elegant way of saying it. I.e., Doctoral/Master of Arts/Bachelor of Arts Dissertation = Ph.D/M.A./B.A. Thesis