Not real or present; something that does not or never existed at all.
Modern English compound: 'non-' (not) plus 'existent' (existing), from Latin 'existere' (to exist). This term became common in the 20th century for philosophical and scientific discussions.
Nonexistent is tricky because you're using a word to describe something that isn't there—philosophers love proving things don't exist by discussing them with this word!
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