a lexicon is the mental dictionary that language users must be equipped with, in addition to the grammatical rules of their language.
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the lexicon includes all the linguistic forms: (regular, irregular, simple, complex, normal, not normal, etc.) that speakers know and use.

neologism When a word that does not exist in the lexicon is created through a morphological rule
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Cutter’ is blocked by the existence of the lexical item ‘knife
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The ‘day after today’ is blocked by the existence of the lexical item ‘tomorrow
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stealer’ is blocked by the existence of ‘thief