What is understood under the term "application"? (Europe, Switzerland) I want to apply for a PhD directly to a professor in a swiss university. In the homepage of his group, under the tab "Open Positions" says "Interested students should send an applicatiobn to ProfessorX".

What is exactly meant with "application" at this point?

An application as I have understood it so far comprises a CV, letter of motivation, note transcript, summary of previous work, references, etc. And most openings I have seen at other universities include a list of such documents to include. However in this university it is quite common to just mention an "application" with no further clarifications or indications.

I am pretty sure if I just send that amount of information in the first e-mail I will just saturate the busy professor.

Another possibility I have thought is that it means a short letter of "introduction/inquiry" where I will shortly explain where, about what and under whom I did my bachelor's and master's thesis. I would also explain my research interests, and maybe include some reference to him/his work/his group. And finally, ask if there is an opening in his group.
It is a vague term and all of the required documents will differ between universities and countries. If you are applying directly to a professor (as opposed to an online form) then he/she should let you know if you are missing anything from your application after you apply.

Also, if the department or professor has a secretary then it might be a good idea to contact them as well to ensure you get all the documents to the right place.