Disjunction introduction is the logic principle that, if A is true, then it's true that either A or B is true.
For example, if it's true that it's raining outside, it's trivially true that either it's raining outside, or my car is freshly waxed. Since a disjunction is true if at least one of the terms is true, and we know that one of the terms is true, the second term is irrelevant for determining the truth value of the disjunction.
Formally:
A
∴ (A ∨ B)
See also: logical disjunction
