Definition:
Features that are targeted for the upcoming release but could be excluded if there are insufficient resources to finalize their development. these are the complement of objectives or requirements that are considered desired or even important to the overall deliverable, but can be considered as optional or nice-to-have in the overall completion of the project.
Further Reading:
Book: The Fifth Discipline Fieldbook by Art Kleiner, Charlotte Roberts, Bryan Smith, Peter M. Senge and Richard Ross