"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, OR, and NOT.
Here is a handy "rule of thumb" for the Boolean operators AND and OR:
- Use AND when you want to find articles that contain ALL terms/concepts. Combining sets with AND will produce a smaller set, since all items in the set must contain all terms.
- Use OR when you want to find articles that have ANY of the terms/concepts. Combining sets with OR will produce a larger set ("OR makes more"), since the items in the set must contain only one of the terms.

For more information about Boolean operators, check out these resources:
- this link from U Mass Boston, which includes a 2-minute video