KL-ONE (pronounced "kay ell won") is a knowledge representation system in the tradition of semantic networks and frames; that is, it is a frame language.
The system is an attempt to overcome semantic indistinctness in semantic network representations and to explicitly represent conceptual information as a structured inheritance network.
One of the innovations that KL-ONE initiated was the use of a deductive classifier, an automated reasoning engine that can validate a frame ontology and deduce new information about the ontology based on the initial information provided by a domain expert.
These form hierarchies using subsume-relations; in the KL-ONE terminology a super class is said to subsume its subclasses.
In KL-ONE descriptions are separated into two basic classes of concepts: primitive and defined.