I was puzzled by the term "dissertation defense" until I found the following in Wiki
One of the requirements for certain advanced degrees is often an oral examination. This examination normally occurs after the dissertation is finished but before it is submitted to the university, and may comprise a presentation by the student and questions posed by an examining committee or jury. In North America, this examination is known as a thesis or dissertation defense, while in the UK and other English-speaking countries it is called a viva voce.
So, looking back at Andrey's question, it would seem that: - dissertation defense is AmE - dissertation defence is CanE (see калина's post 4 above)
- for other varieties of English you'd choose a different term altogether