In logic and mathematics, a disjunction is an "or statement". For example "John skis or Sally swims" is a disjunction.

Note that in everyday language, use of the word "or" can sometimes mean "either, but not both" (eg, "would you like tea or coffee?"). In logic, this is called an "exclusive disjunction or "exclusive or". When used formally, "or" allows for both parts of the or statement (its disjuncts) to be true ("and/or"), therefore "or" is also called inclusive disjunction.

The statement "P or Q" is often written as

Such a disjunction is false if both P and Q are false. In all other cases it is true.

For two inputs A and B, the truth table of the function is as follows.

A B | A or B
F F |    F
F T |    T
T F |    T
T T |    T

More generally a disjunction is a logical formula that can have one or more literals separated only by ORs. A single literal is often considered to be a degenerate disjunction.

For example, all the following are disjunctions:

A ∨ ¬B ∨ ¬CD ∨ ¬E

The equivalent notion in set theory is the set theoretic union.

See also