"Boolean operators" are the connector words that are used to combine sets of search results to achieve certain desired effects. They are named for George Boole, a 19th century English mathematician. The most commonly used Boolean operators are AND and OR.
Here is a handy "rule of thumb" for the Boolean operators AND and OR:
For more information about Boolean operators, check out these resources:
The Boolean operator NOT may be used to exclude terms from your search. At times it is useful, but it must be used very cautiously, as it can inadvertently remove results that you actually wanted to keep. Practically speaking, the Boolean NOT is not used very often.
For example, in MEDLINE you might be tempted to use NOT to eliminate articles with the Subject Heading Animals, but in the process, you could be losing valuable articles that have both the Humans and Animals tags.
Consider this citation from Ovid MEDLINE:
Here is the list of Subject Headings assigned to that article:
If you had "NOT'ed" out all articles with the Animals search tag, you would have missed this article.
What the Boolean operator NOT is good for is subtracting out the results of a set that you already examined.
Suppose you ran a search on the MeSH headings Zika Virus AND United States. You painstakingly examined all 205 results in Set 3, ultimately keeping only 12 of them (Set 4). Then you decided to repeat the search using the MeSH headings Zika Virus Infection and United States. This time you come up with 324 results (Set 6), but you suspect many of those were included in Set 3, which you already looked at. So you decide to save yourself some work by "NOT'ing" out Set 3. Now you only have to look at 128 results (Set 7).