Logical disjunction

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").

The statement "P or Q" is often written as

P ∨ Q

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

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
¬A ∨ B
A ∨ ¬B ∨ ¬C ∨ D ∨ ¬E

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

See also

• Logical conjunction
• Existential quantification
• xor
• nor