Definition:
Rational Unified process is an object-oriented and Web-enabled program development methodology. according to Rational , RUP is like an online mentor that provides guidelines, templates, and examples for all aspects and stages of program development. RUP and similar product — like Object-Oriented software process , and the OPEN process — are comprehensive software engineering tools that mix the procedural aspects of development with different components of development within a unifying framework.
Further Reading:
Book: Writing Effective Use cases by Alistair Cockburn.