Broader and more narrow terms.
As you begin your research, it is good to brainstorm terms. Think about terms that are broader or more narrow, synonyms, etc.. This is particularly useful for subject areas that are emerging/new, such as technology.
Even if you start with keyword searching, once you start finding materials that you think are on point with your topic, you can take the relevant subject headings and use them to find additional materials specifically on your topic.