This single word expresses agreement, affirmation, or acceptance. It is often used as a response to a question or request.
From Wikipedia
Yes and no, or similar word pairs, are expressions of the affirmative and the negative, respectively, in several languages, including English. Some languages make a distinction between answers to affirmative versus negative questions and may have three-form or four-form systems. English originally used a four-form system up to and including Early Middle English. Modern English uses a two-form system consisting of yes and no. It exists in many facets of communication, such as: eye blink communication, head movements, Morse code, and sign language. Some languages, such as Latin, do not have yes–no word systems.