I read Being and Time a few years ago, but it seems to me the question is only partially answered there. He deals with the question of whether Being is "indefinable" very early on [p 4]. He claims that Being is not an entity, and so not amenable to traditional analysis or definition as in "logic". An early section though claims it is essential to any meaning whatsoever, so he urges us to "look that question in the face". Is this question answered elsewhere in Heidegger? What is the answer to Heidegger's question "what is the meaning of Being"?
The opening section of Being and Time tells us that being is not a concept, which means it will not admit of the same sort of definition that a concept like triangle might. Nonetheless, being is that on the basis of which beings are understood as such, that is, always and everywhere when we deal with beings in any way we necessarily approach them on the basis of being, with a non-conceptual preconception of what being is such that they can be what they are and how they are. Thus, despite being’s indefinability, it is the most ubiquitous and everyday thing, which is what guides Heidegger’s approach to the question.
All of Being and Time is a response to the question “What is the meaning of being?” The first division reaches the conclusion that the meaning of being is care. Care has a three-fold structure: “the Being of Dasein means ahead-of-itself-Being-already-in-(the-world) as Being-alongside (entities encountered within-the-world).” This structure stems from the three primordial forms of openness of Dasein, which have their foundation in the three dimensions of time.
Being-ahead-of-itself relates to understanding, by which we project our own potentiality-for-being, and the possibilities of the things we encounter in the world. This is primarily futural. Being-already-in-the-world relates to our disposition, which is a way of finding ourselves already attuned to everything in the world, not by virtue of a conscious decision, but always already, by virtue of what Heidegger calls our thrownness. This is primarily past-oriented. And Being-alongside (entities encountered within-the-world) expresses the propensity for everything in our world to show up as what it is, for us to accept everything as it is defined by everyday discourse without truly attending to the being or essence of what presents itself. Heidegger calls this falling and it is primarily related to the past.
So the phrase “the meaning of being is care” means that the being of everything we encounter has its foundation in our care, which has the three aspects listed above. That these three aspects are grounded in the temporalizing of Dasein means that the meaning or foundation of both being and care ultimately rests on time. So the meaning of being, and by extension the foundation of all beings, is time, which causes Dasein to be thrown out of its past, to fall through its present, and to project its future. along these three axes everything in our world is determined as what it is.
This is not yet a complete answer to the question of the meaning of being, however. Being and Time is incomplete, and it ends with Heidegger saying precisely that the question of the meaning of being has not yet been answered. Heidegger’s later writings make a number of criticisms of his approach in Being and Time and offer new insights into the question. Nevertheless, a diligent reading of his magnum opus should see the entire work as the approach to this question.
His later writing transitions from the temporality of being to the epochality of being. He comes to see the approach of Being and Time as too enmeshed in the traditional philosophical idea of the subject, as it attempted to understand being starting from the being of the human being (Dasein) rather than seeing Dasein as part of a broader context including language, thinking, being, and things or earth, sky, divinities, and mortals. Some of his later writings, including “Building, Dwelling, Thinking” and “The Thing” (both in Poetry, Language, Thought) are worth reading to pursue this development.
He also takes issue with his use of the phrase “fundamental ontology” in Being and Time. A fundamental ontology would be a discourse which spoke of being directly, without mediation, but Heidegger comes to see that this is impossible. So he further develops the idea of the epochality of being, that being has passed through epochs in which it was predominantly understood in a certain way by the majority of thinkers in the philosophical tradition. This recognizes the implicit historicality of being - being can only come to be historically and for historical beings (such as human beings), so it cannot be understood directly, as Being and Time attempted to do, but must be understood through the medium of history. I have written more about how Heidegger approaches the history of philosophy here: What is the difference between Derrida's Deconstruction and Heidegger's Destruktion?
If you are interested in reading more about this aspect of his self-critique, I recommend the “Letter On Humanism” in Pathmarks and On Time and Being. He never comes to what could simply and straightforwardly be called a definition, primarily because definitions offer concepts (genus and specific difference) while being is not a concept. Nevertheless, everything he wrote including Being and Time should be read as being in pursuit of this question.