The explanation of what a PhD thesis isn’t clear because the term "dissertation" is more often used. Even then, understanding the distinction between dissertation and thesis isn’t easy. Sometimes to get a PhD, people write a thesis and other times they write a dissertation, and the terms may be switched elsewhere. In places like the US, the convention is that students gaining a master’s degree write a thesis and PhD candidates write a dissertation. Such complexities are best understood by explaining that the written production of the PhD student is generally more complex, longer, and has several key features.
US readers, when they see the term PhD thesis, should refer to it as a PhD dissertation for easiest understanding. In PhD programs, a lengthy written or researched project is key to earning a degree. Length may vary per school requirement, but it’s not uncommon to need to write about 200-300 typed pages of material. This material is a thorough treatment of a topic in some way that is novel and that contains original research so that new scholarship is added to the topic or something is discovered that was hitherto unknown.
Not only does a PhD thesis need to be logical, present something new, and add to scholarship on an issue, but it must show knowledge in all present and past scholarship on the issue. A tremendous amount of research is required, and once the thesis/dissertation is completed, the faculty reviews it. A student will have to defend it orally by taking questions, and it is only by the decision of faculty that the dissertation is accepted.
Not all people are successful when they first write a dissertation. They have to rewrite, do more research or possibly throw out a topic and start over. Having a supportive faculty advisor to help with dissertation formation is very useful to avoid this.
The features of the master’s thesis, which is sometimes confusingly called a dissertation, are different. This work is normally a shorter one, less than 100 pages, and not all master’s programs have a thesis requirement. The thesis is usually a novel or interesting approach to a topic that doesn’t require original research. People writing a thesis need to have strong knowledge of all prevailing research on their topic and they can use this knowledge to present supporting arguments. As with the PhD thesis, people presenting a master’s thesis usually have it reviewed by faculty members and must defend their thesis in a public setting.
The complexity of the PhD thesis or dissertation explains why doctoral studies can take so long to finish. In order to really perform original research, people may need to examine documents that are at a distance, create original clinical projects, or in other ways spend time creating a piece of original scholarship. The length and rigors of research required may take several years to complete, and some people will take as many as four years or more after finishing classes before they are able to present a successful dissertation.