disjunctive syllogism
From "P or Q" and not P, conclude Q.
Disjunctive syllogism is the valid form: P or Q; not P; therefore Q. It is the process of elimination. The second premise must deny a disjunct; asserting one (affirming a disjunct) licenses nothing about the other.
Examples
"The key is in my coat or in the car. It isn't in my coat. So it's in the car."
See also