Is this a first kiss?

I am pretty ignorant in relationship. What is consider a first kiss? Does it have to be mutual (mouth to mouth)? Or, can it be one-side (mouth to other part of the body)? I was offered to be kissed by a very handsome boy, but I did not say "Yes" at the spot. He was gentleman enough that he later offered me a goodbye kiss at my cheek. I was never kissed by a boy before, so was that my "first kiss" already? So, confused!!