The objects, concepts and relationships that are assumed to exist in some area of interest. A collection of knowledge, represented using some knowledge representation language is known as a knowledge base and a program for extending and/or querying a knowledge base is a knowledge-based system.
Knowledge differs from data or information in that new knowledge may be created from existing knowledge using logical inference. If information is data plus meaning then knowledge is information plus processing.
For example, a knowledge base about a family might contain the facts that John is David's son and Tom is John's son and the rule that the son of someone's son is their grandson. From this knowledge it could infer the new fact that Tom is David's grandson.