Никогда не сталкивалась с такой простой лексикой и боюсь, что здесь есть свои особенности… Please help!!

thesis это, конечно, диссертация. Не путайте.

Indeed, one has a lot of ground to cover in order to progress from an undergraduate thesis (also called bachelor’s theses) to a doctoral dissertation (or thesis); however, it does not mean that both research papers cannot be called “thesis”. Have you ever tried to look at graduation requirements of major universities (preferably western ones, Russian schools have a peculiar habit of “coining” their own terms)? Well, if you do, you will notice that in order to graduate with a Bachelor Degree, one often has to submit a Bachelor's Thesis (sometimes also called –surprise! -Bachelor’s dissertation); for a Masters Degree, however, one has to write a Mater’s theses/dissertation; and if the pesky little fellow we are talking about is determined to get a doctoral degree, poor bugger has to slave over a PhD thesis/dissertation.

REQUIREMENTS FOR GRADUATION from Connell Universiy
The Bachelor's Thesis
A Bachelor's Thesis on a selected topic in your chosen specialty area is required for graduation. The selection topic will become your undergraduate major and will be included on your student's final transcript. Please consult with your Mentor or refer to your Student Handbook for further details. International students may write their Theses in their native languages. - http://www.connelluniversity.org.uk/gra.htm

From the University of Maine web page:
Graduation Requirements for the M.S. Degree
The M.S. degree is offered in both thesis and non-thesis options, with thesis and project topics available in the areas of current faculty research. Through their thesis work and assistantship responsibilities, graduate students play an integral part in both the research and instructional goals of the department. http://gradcatalog.umaine.edu/preview_program.php?catoid=17&poid=1583

Some other helpful links
http://www.physics.ucsb.edu/~kerri/ugradweb/honors.html
http://www.honors.ufl.edu/upperdivisionhonors.html
http://www.scs.uiuc.edu/chem/u_research/499thesis.html

“A University Diploma, which is equivalent to the first year of the relevant Bachelor's degree program, is designed for students who have completed senior high school but may not quite meet the academic and/or English entry requirements for a university degree program.
The University Diploma Programs are two semester programs of 13 weeks plus examination period each semester with three new intakes every year. “
http://www.qutic.qut.edu.au/courses/diplomacourses/

“The University Diploma in Arts is designed for students who wish to obtain an intermediate credential that provides a grounding in the intellectual skills required of university studies. It is also a foundation for further studies in Humanities and Social Science. …
Students who have completed a more senior-level credential (e.g., bachelor’s degree) may not enrol in the University Diploma in Arts program.”
http://www.athabascau.ca/calendar/page03_22.html

Русский инженерный Диплом is a document confirming a degree (something in-between Bachelor’s and Masters Degrees, depending on the university). So, by translating “диплом” as diploma, you are striping a qualified specialist of his hard-earned degree! With his credentials translated as diploma, he will be looked upon as an engineering technician.

Подмена понятий, взятых из разных систем координат – is exactly what you, translators/interpreters, do for a living. Often, substitution is the only way to get understood in our different worlds with different coordinate systems without full linguistic and cultural equivalents.So, while translating you have to find “the closest natural equivalent to the source-language message”. And this is My Personal Truth.