A good comprehensive search strategy includes two kinds of search terms: keywords (any words or phrases that you choose) and subject headings (specific terms created by the database to represent particular concepts). Having multiple search terms for each of your concepts ensures that your search strategy captures all literature relevant to your question. The goal of a systematic review is to retrieve a comprehensive collection of literature, not just from authors who'd have used the same words, word forms, spelling and hyphenization as you.
If you feel stuck, there are several sources available to help you discover additional search terms:
Commonly Utilized* in Systematic Review Searches
*Some publishers require searching of particular databases for systematic reviews, check the requirements of your target publishing venue
Specialized Databases (consult a librarian for recommendations)
"Grey literature" refers to formats that are not published in the traditional book/journal medium or indexed in bibliographic databases.
Studies have shown that articles with positive results from respected institutions by well-known authors in English or from a developed country are more likely to be published than other articles. Seeking grey literature helps ensure that the results of a comprehensive review are as objective and free from of this publication bias as possible. Formats include:
DISSERTATIONS & THESES
MISCELLANEOUS GREY LIT SOURCES