Determining what is relevant can be an incredible task. It is the goal of all search engines to figure out what is the most relevant Web pages for that particular searcher based on the one, two, three or four words they type into the query box. It is not easy because of the subjectivity involved in the query process.
A thread at Search Engine Watch Forums named What is Relevancy discusses just that. Orion, the resident search technologists, gives the text book scientific definition, "A judgment which relies heavily on semantics." But then he goes deeper into how someone in the information retrieval field would determine such a thing. He says you set a hypothesis, such as "If a document and a query have a (key)word in common, the document is likely to be relevant." And then you try to disprove it, he adds, "Some IR systems precisely are designed for the sole purpose of disproving this hypothesis."
Mel then goes into the subjectivity of such a question asking; "Relevancy to the words searched for in a search engine? Relevancy to the topic searched for in a search engine? Relevancy regarding links? Relvancy for the searcher?" and so on. There are others the chime in to the thread, such as ProjectPHP. And then Danny Sullivan gives a great recap at this post.