adjective / noun / interjection
affirmative
uh-FER-muh-tiv/əˈfɜːrmətɪv/
Expressing agreement or confirmation.
The nod and raised thumb give an affirmative answer.

Etymology
The underlying idea of making firm became confirming or asserting something. In logic, an affirmative answer asserted that something was so—the basis of its meaning ‘yes’ or agreement.
In context
even opening an investigation requires the affirmative votes of four commissioners.