Princeton's WordNet

WordNet is a large lexical database of English. Nouns, verbs, adjectives, and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept. Synsets are interlinked by means of conceptual-semantic and lexical relations. The resulting network of meaningfully related words and concepts can be navigated with the browser and also freely available for download. WordNet's structure makes it a useful tool for computational linguistics and natural language processing.

https://wordnet.princeton.edu/download