A patient with a longstanding psychotic illness repeatedly uses the word 'plimtosis' to describe a private, hard-to-put-into-words inner experience, using it consistently across several conversations. When asked, he offers an explanation of what he means by it, though the word itself is not a real one. What is this?

This is a neologism: a word that is either wholly new or an existing word used with an idiosyncratic, non-standard meaning, here specifically a technical neologism coined for a private, otherwise indescribable experience, which is why he can explain what it means. It is not a paraphasia, which arises from a language disorder such as aphasia rather than from psychotic thought content. It is not a malapropism, which is a conspicuously misused but ordinary word, generally without particular psychiatric significance, whereas 'plimtosis' is not an existing word at all. And it is not verbigeration, which is continuous repetition of the same fixed content, not the coining of a single new term.