As you conduct research, you are likely to realise that the topic that you have focused on is more complex than you realised when you first defined your research question. The research is still valid even though you are now aware of the greater size and complexity of the problem. A crucial skill of the researcher is to define clearly the boundaries of their research and to stick to them . You may need to refer to wider concerns; to a related field of literature; or to alternative methodology; but you must not be diverted into spending too much time investigating relevant, related, but distinctly separate fields.
Generally in the US, a thesis is the final project for the masters degree and a dissertation leads to a doctoral degree. Those pursuing a masters degree must perform research on a specific subject that demonstrates their knowledge acquired through their program. Seeking a PhD is different in that your dissertation must contribute something completely new and undiscovered to your field. In other words, you have to contribute original knowledge to the subject. So the main difference between a thesis and a dissertation is the depth of knowledge you must attain in order to write the paper.