Definition:
The Open Unified Process (OpenUP) is a part of the Eclipse Process Framework (EPF), an open source process framework developed within the Eclipse Foundation.
Its goals are to make it easy to adopt the core of the Rational Unified Process (RUP). OpenUP preserves the essential characteristics which include iterative development, use cases and scenarios driving development, risk management, and architecture-centric approach. Most optional parts of RUP have been excluded, and many elements have been merged. The result is a much simpler process that is still true to RUP principles.
OpenUP targets small and collocated teams interested in agile and iterative development. Small projects constitute teams of 3 to 6 people and involve 3 to 6 months of development effort.
Further Reading:
BOOK: Disciplined Agile Delivery by Scott W. Ambler and Mark Lines