true and false

There are two constants used to represent truth and falsity: true and false. false is defined as 0 (zero).
true is often said to be defined as 1, which is correct, but true has a wider definition: any integer which is non-zero is true, in a Boolean sense. So -1, 2 and -200 are all defined as true, too, in a Boolean sense.
Note that the true and false constants are typed in lowercase unlike HIGH, LOW, INPUT, and OUTPUT.