once i was told that over 160 definition exist for relevance. tefko saracevic has more to tell about it. and he knows the history of that notion in information retrival:

• What are the manifestationsof relevance? • What is the behaviorof relevance? Or more precisely: How do people behave in respect to relevance? • What are the effects of relevance? Or more precisely: What influencesare related to relevance? | Part II: Nature and Manifestations of Relevance*