1) the act of acknowledging (acknowledgment)
(of) a statement expressing 'Yes', agreement etc or one that is true, real etc as in 'The answer is (in the) affirmative
being agreeable or assenting
an arrangement or understanding (spoken or written) made by two or more people, governments etc
concurrence
the act of recognizing or being recognized
identification, acknowledgment
reacting easily or quickly
quick to react