Also: diploma work

thesis is a written paper (based on research, of course). Sometimes it is not a paper, but a project that leads to the degree. So it depends what it actually is - a paper or a project.
In English-speaking countries, Diploma is not what people get after University, it is more like something awarded after a college, a technical school, etc, and then can be considered as prior learning to reduce the number of years at the Uni. University normally results in a Degree - Bachelor (usually 4 years full-time) or Masters (usually 5 years.
So, I guess the choice of words will depend on 1)is it a paper or a project 2) on the type of the educational establishment/course.
I hope this is helpful
best of luck to you