Refers to the vocabulary component of a language, often in linguistic or computational contexts. Not a physical dictionary.
The lexicon of this language consists of about 50,000 words.
辞書 means 'dictionary' (a reference book), while 辞書部門 refers to the abstract set of words (lexicon) of a language.
語彙 is the common word for 'vocabulary' or 'lexicon'. 辞書部門 is a more technical term used in linguistics.
Compound of 辞書 (dictionary) and 部門 (division, category). The term is a calque from English 'lexicon' in its linguistic sense.