dic·tion·ar·y /ˈdikSHəˌnerē/ noun a book or electronic resource that lists the words of a language (typically in alphabetical order) and gives their meaning, or gives the equivalent words in a different language, often also providing information about pronunciation, origin, and usage.

Many dictionaries aim to be descriptive, not prescriptive, as doctor_eval said. That definition doesn't change that:

OED: "The Oxford English Dictionary is not an arbiter of proper usage, despite its widespread reputation to the contrary. The Dictionary is intended to be descriptive, not prescriptive. In other words, its content should be viewed as an objective reflection of English language usage, not a subjective collection of usage 'dos' and 'don'ts'. However, it does include information on which usages are, or have been, popularly regarded as 'incorrect'."[1]

Merriam-Webster: "Merriam-Webster is a descriptive dictionary in that it aims to describe and indicate how words are actually used by English speakers and writers. Generally, the descriptive approach to lexicography does not dictate how words should be used or set forth rules of "correctness," unlike the prescriptive approach."[2]

OUP: "All of Oxford Languages' content aims to describe, rather than prescribe, the way languages are used by people around the world."[3]

I believe your definition comes from one of the Oxford dictionaries - iOS lists it as coming from the Oxford Dictionary of English, and Lexico (a partnership with Oxford, whose About page links to [3]) includes it[4].

[1] https://www.oed.com/public/oed3guide/guide-to-the-third-edit...

[2] https://www.merriam-webster.com/words-at-play/descriptive-vs...

[3] https://languages.oup.com/about-us/how-we-create-language-co...

[4] https://www.lexico.com/definition/dictionary