Oneness is a peaceful feeling of being part of a whole — of not standing off to the side. People use it when they feel joined to other people, to nature, or to something larger than themselves. It is a feeling word, not a maths word. You are not counting one. You are talking about being one with. You will hear it in religion, in quiet practice like meditation, in poems, and in talk about love or community. It is soft and serious. You would not throw it at a football win unless you wanted to sound grand. Though actually, the older, cooler meaning is just unity: being a single thing, or being in agreement. That meaning is still possible, but the warm, joined-together feeling is the one most learners will meet.