A lexicon is an individual's personal stockpile of knowledge about words including their meanings, pronunciation, and syntactic roles.
Vocabulary: Vocabulary refers to all the words known or used by a particular person.
Semantic memory is our memory for facts or general knowledge about the world. It includes our knowledge about meanings of words we learned from our lexicon.
Proposed by Noam Chomsky, LAD is an innate mechanism or process that facilitates children's learning of grammar and syntax in their native languages.