A Build Package is the resulting artifact of a software build process in an Automation Pipeline. This can be, for example, a Docker image, a compressed archive file or anything else that encapsulates the whole software system including all software dependencies. It usually encapsulates a specific Release Version. Sometimes the terms are used synonymously.
For executable software components - as oppose to libraries - the Build Package contains everything that is needed to run the software in a Runtime Environment. In that context a Build Package is especially intended can run in any available environment - be that resting, staging or production.