What is a Brain?
A brain is the physical organ, typically held within the skull, that generates and supports the mind and is studied by neuroscientists, biologists, physicians, and many others.
Notes
I don't attempt to define the brain too completely, and I'll explain why below.
A literal definition of the brain might describe the neuron-based mass that is found in the skull of many species of animal, including human beings.
But there are other constituents of the brain such as glial cells, the neuroimmune system, hormones, neurotransmitters, and other chemical that can impact the functioning of the brain as a whole.
In addition, the workings of the brain are not restricted to the skull as the nervous system extends out to all extremities in the body.
And even further than that, there are connections (both neuronal and chemical) to the body, and in particular the gut, the gut and body biome, the body immune system, the heart and major organs, and all other physical parts of the body. These are not, generally, physically located in the brain but do influence the workings of the brain.
To keep this very complex set of appreciations simple, I adopt a consciously vague definition of the brain as given above.