Specification pattern is a reusable and object-oriented way of applying business rules on domain entities. The primary use of specification pattern is to select subset of entities from a larger collection of entities based on some rules. An important characteristic of specification pattern is combining multiple rules by chaining them together.

Specification pattern was in existence before ORMs and other data access patterns had set their feet in the development community. The original form of specification pattern dealt with in-memory collections of entities. The pattern was then adopted to work with ORMs such as NHibernate as people started seeing the benefits that specification pattern could bring about. We would first discuss specification pattern in its original form. That would give us a good understanding of the pattern. We would then modify the implementation to make it fit with NHibernate.