A lexicon is an individual’s general store of word and phrases – essentially a “mental dictionary.” Almost every career or vocation has a “lexicon” of the vocabulary that is specific to that field.
For instance, think about some hobby or interest that you have and the words that have specific and possibly unique meanings that relate to that field. Think about some of the words that psychologists use with each other- research design, placebo, treatment condition, neuroleptic, SSRI, etc. These are part of a psychologist’s lexicon- although most psychologists would be very familiar with these words someone who studies calculus may not know what any of these terms mean.