Domains are autonomous, but they rely on the existence of other domains. For example, an online bookstore domain can be modeled without reference to any mechanism that selects the item the customer wants or the desired quantity. The user interface mechanism could be HTML, voice recognition software, or some other yet-to-be-invented technology.
We implicitly assume that there is some way to determine the specific items and quantities of them to add to the order. This implicit assumption in turn embodies requirements that somehow the customer can communicate with the bookstore application. A bridge exists between the bookstore domain and some other domain that can satisfy the requirement implied by the assumption.
Definition: A bridge ...