Being is the quality or state of having existence. Specifically this is the gerund form of the verb to be which functions as a substantive (noun).
As a noun, being may refer to either a thing or a substance. The latter is really our concern. A being in this sense is something that is generally living, or if not living, then conscious (certain substances may be conscious, but not living in an embodied state, and some substances may be living—plants, for example—but not conscious). A substance is the owner and unifier of properties, parts, and capacities. A being remains absolutely the same through law-like, repeatable change to reach a final stage if change is part of its nature. Each being or substance has a nature or secondary substance to it. For example, John Doe is a particular being with a human nature. 'Oneness in nature' does not entail 'oneness in being'. John, Joe, and Jack are one in their nature (i.e., they are all human), but they are not one in their being (i.e., they are all separate human beings). The members of the Trinity are not only one in the former sense, they are one in the latter sense (i.e., they are all inseparably united as one being).