When we say a system is Boolean, we mean it only has two sides: true or false, yes or no, on or off — never a soft “maybe” in the middle. Computers are built on this idea, because a chip is really a huge set of tiny switches that are either on or off. Programmers use Boolean values all day, and they join statements with little linking words called operators: AND (both must be true), OR (at least one is true), and NOT (flip true to false, or false to true). Internet search uses the same trick: “cats AND dogs” keeps only pages with both; “cats NOT dogs” throws the dogs out. The word can be an adjective (Boolean logic) or a noun (a Boolean). Careful English often keeps the capital B, because it is a person’s name; in code you will usually see boolean in small letters. Though actually, people stretch it in everyday talk to mean any blunt yes/no test, even far from computers.