Merriam-Webster Collegiate Dictionary - yes

 I. adverb  Etymology: Middle English, from Old English gese  Date: before 12th century  1. — used as a function word to express assent or agreement are you ready? Yes, I am  2. — used as a function word usually to introduce correction or contradiction of a negative assertion or direction don't say that! Yes, I will  3. — used as a function word to introduce a more emphatic or explicit phrase  4. — used as a function word to indicate uncertainty or polite interest or attentiveness  II. noun  Date: 1712 an affirmative reply ; yea