Usually a lexicon is a container for words belonging to the same language. Some exceptions may be encountered for languages that are variants, like for instance Brazilian Portuguese compared to European Portuguese, where a lot of words are common and where the differences may be marked word by word.
When linguists study the lexicon, they study such things as what words are, how the vocabulary in a language is structured, how people use and store words, how they learn words, the history and evolution of words (i.e. etymology), types of relationships between words as well as how words were created.
An individual's mental lexicon, lexical knowledge, or lexical concept is that person's knowledge of vocabulary. The role the mental lexicon plays in speech perception and production, as well as questions of how words from the lexicon are accessed, is a major topic in the fields of psycholinguistics and neurolinguistics, where models such as the cohort model have been proposed to explain how words in the lexicon are retrieved.