We then extract keyphrases from the retrieved passages based on topic signature words (Lin and Hovy, 2000)   words, together with their related terms from WordNet (Miller, 1994), are used to determine whether a phrase in the passage is a keyphrase.
a keyphrase is (1) a noun phrase or verb phrase that is shorter than 10 tokens; (2) contains at least one content word; (3) has a topic signature or a Wikipedia title.