The lexicon of a language is vast, irregular, and open-ended; yet human language users effortlessly store and retrieve tens of thousands of words along with knowledge about the concepts they express. Meaning similarity is a major principle in the organization of the mental lexicon. While traditional dictionaries do not reflect this, the digital resource WordNet interconnects words based on their semantic relatedness. The resultant nework can be navigated by computers to detect and measure meaning similarity. WordNet is used in many applications that attempt to process language and understand both overtly expressed and implied meaning. WordNet's use in natural language understanding provides a testing ground for theories of the lexicon and lexicalization patterns. WordNets that were subsequently built in dozens of unrelated language yield a perspective on the universality of concepts and their linguistic encoding.