affirm

Short definition from Wiktionary

Verb

1. 1
   To agree, verify or concur; to answer positively.
2. 2
   To assert positively; to tell with confidence; to aver; to maintain as true.
3. 3
   To support or encourage.
4. 4
   To make firm; to confirm, or ratify; especially (law) to assert or confirm, as a judgment, decree, or order, brought before an appellate court for review.
5. 5
   To state under a solemn promise to tell the truth which is considered legally equivalent to an oath, especially of those who have religious or other moral objections to swearing oaths; also solemnly affirm.

Interjection

1. 1
   Yes; true; correct.