As a traditionally object medium, exhibits can embrace all media; word, image, moving image, sound, performance, installation, plants and animals, digital technologies, etc. But the most persistent definition of "exhibit" is that it is inherently physical; it has dimension, takes up space, and has (hopefully real) stuff. As a form of communication, it's a narrative you can walk around in. It's both a noun and a verb. To actually touch, or to be touched by something. To experience directly and to make meaning for yourself, alone or socially. It's an active (hopefully interactive) full body sensory exploration of content in a physical space.
The elusive line between exhibits in museums and in the world at large is interesting. One one hand, a well designed museum exhibit is an experience of "embedded knowledge", where content is integrated in a meaningful way in every aspect of the space itself. Museums are the traditional keepers of sophisticated exhibit design research and best practices. On the other hand, from private living rooms to the streets, the world is one huge informal exhibit, unconsciously or intuitively or experimentally designed. Out there we can learn about communication and meaning making in all it's forms, as if "exhibit" can be defined as any time we say look! there's an important story here.