Think of keywords as something the author said, in exactly the way they said it. Computers accept what you type in a literal fashion. When you search, consider:
Improve your search strategy by using Boolean search operators ("and", "or", "not"). They allow you to be more specific in how you combine search terms.
Use AND to focus search and combine different aspects of your topic.
Example: hematopoiesis and simulation
Use OR to expand your search and find synonyms/related terms.
Example: refinement or alternative
Use NOT to exclude a word or phrase from your search
Example: dogs not mice
Phrase search - Use quotation marks (" ") to search for a particular phrase.
Example: "tissue culture"
Truncation - Use an asterisk (*) or question mark (?) to find variations of a word. Put an asterisk following the root of the word to find all variations of that word, including singular and plural.
Example: monitor* (finds: monitor, monitors, monitored, monitoring, etc.)
Grouping or Nesting Keywords - Use parentheses ( ) as a way to group all your search terms together.
Example: ("surgical stripping" or "surgical ligation") and (anesthe? or anasthe? or anaesthe?) and method?
The search process needs to be documented in enough detail throughout the process to ensure that it can be reported correctly in the review, to the extent that all the searches of all the databases are reproducible.
Cochrane Handbook for Systematic Reviews of Interventions, version 5.1.0 (2011). Section 6.6