Break your research question into its main concepts. Determine synonyms for each concept and add new ones as your search evolves. Download the Yale concept table or make your own grid to keep track of terms.
Subject headings and Textwords
Look for subject headings in the indexing of useful articles, and note textwords that occur in relevant and irrelevant results.
Subject headings help to find articles on a similar topic that may use very different terminology such as synonyms, plurals, or Britishisms. Scroll down on this page to see an example of some of the Entry terms found by using the single subject heading word Neoplasms in PubMed, including all the variant terms such as cancer, tumor/tumour/tumours, malignancy, etc.
The same page demonstrates that in some databases like PubMed, subject headings are hierarchical, so searching the broadest term can also search the more specific terms in the hierarchy, such as different types of neoplastic cells and neoplasms by body location. In MeSH, one can also choose to focus on the Major subject headings, the two or three concepts that are most important in that article. MeSH also provides subheadings that further define an aspect of the subject, such as its etiology or treatment.
Thesauri: The page mentioned above is an example of a thesaurus, a dictionary of standardized terms used to index documents in that database that may show synonyms and definitions.
Related article tools: Scopus, Web of Science and Google Scholar do not have subject headings other than terms that may be imported from other sources or assigned by the author. These may use a See Related/Find Similar, etc. feature that uses co-citation networks to identify related articles (inferring that articles that cite the same references describe similar topics). PubMed's Similar Articles feature is based on title, abstract and subject heading terms.
Importance of textwords
Use textwords to find publications in databases that don't have subject headings, to find new concepts that don't yet have subject headings (such as new diseases, treatments or societal concepts), articles that indexers have used variant terms to index, and as a quality control measure of the thoroughness of your search strategy. In PubMed, many newly added articles have not yet been indexed so won't be included in results using only subject headings. To focus results, you can try searching for these terms in the title, abstract or subject heading fields.
Indexers may vary on how they apply search terms, so a good search uses a mixture of subject headings and textwords.
Combine your search terms with operators. Some databases require that they be capitalized (Articles+, Google Scholar)
|AND||finds items with BOTH terms, use for intersection of two ideas|
|OR||finds items with EITHER term, use for synonymous ideas|
|NOT||excludes items with term; use with care, useful for reviewing changed search strategy results|
|phrase searching||usually quotation marks, can break mapping to subject headings, usually not available with truncation|
|truncation||usually *, sometimes ?,finds word variants that begin with the same root, breaks mapping in PubMed, not used in Scholar|
|wildcards||used to replace one or more letters in a word|
|word order||adjacency within a few words, words in particular order. for example term1 adj3 term2 in Ovid|
|processing order||PubMed processes from left to right; with PubMed and most databases using parentheses can determine the order of processing. others like Scholar process by operator.|
Geographic locations - hierarchy of continents, regions and nations
Study types - hierarchy of different study types, including variants of epidemiologic studies
Animals - hierarchy of animals by population group and order/species
Ecological and Environmental Phenomena - environmental, climactic and meteorological terms (note Environment includes Microbiota)