WordNet is a lexical database from Princeton. It consists of nouns, verbs, adjectives and adverbs which are grouped into sets of synonyms (synsets), hyponyms, meronyms and hypernyms with descriptions. Each synset describes a concept and is interlinked by means of conceptual-semantic and lexical relations.
Similar structured knowledge bases are ConceptNet or VerbNet. These focus on the major languages, but there are also initiatives for smaller languages like the Open Dutch WordNet. Applications can be tasks for word sense disambiguation, classification of texts, finding similar terms, lexical simplification, etc.
This article is part of the project Periodic Table of NLP Tasks. Click to read more about the making of the Periodic Table and the project to systemize NLP tasks.