Why is there an entry for the word 'dictionary' in a dictionary?

dictionary

dic⋅tion⋅ar⋅y   /ˈdɪkʃəˌnɛri/ Show Spelled Pronunciation [dik-shuh-ner-ee] Show IPA Use dictionary in a Sentence See web results for dictionary See images of dictionary –noun, plural -ar⋅ies.

1. a book containing a selection of the words of a language, usually arranged alphabetically, giving information about their meanings, pronunciations, etymologies, inflected forms, etc., expressed in either the same or another language; lexicon; glossary: a dictionary of English; a Japanese-English dictionary.
2. a book giving information on particular subjects or on a particular class of words, names, or facts, usually arranged alphabetically: a biographical dictionary; a dictionary of mathematics.
3. Computers. a. a list of codes, terms, keys, etc., and their meanings, used by a computer program or system. b. a list of words used by a word-processing program as the standard against which to check the spelling of text entered.

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