In Logic the term ‘disjunction’ is used to designate one of five functions in Propositional Logic where at least one atomic propositions that compose a compound proposition is true.

For example, take the following two simple propositions:

  1. Kato is a dog.
  2. Kato is a mammal.
These two simple (or atomic) propositions can be disjoined to form the compound proposition, Kato is a dog or Kato is a mammal. By disjoining the simple propositions into a single compound proposition we are claiming that one of the elements is true (and it does not matter which).

Every proposition, whether simple or compound, has a truth value, the disjunction of two propositions will be true in all cases but one: when neither component is true.

In ordinary language there are different words that serve the logical function of disjunction: ‘or’ and ‘unless’, etc.