a pre-installed list of words makes up the dictionary service. These are typically localized and users can add words and override the iOS and macOS dictionaries.
The Oxford Dictionary of English defines “dictionary” as “a book that lists the words of a language in alphabetical order and gives their meaning, or that gives the equivalent words in a different language.“, alternatively “a reference book on any subject, the items of which are arranged in alphabetical order”. In the context of Ask Different, this tag covers the lookup software provided by Apple’s operating systems (i.e. Dictionary.app in macOS, the dictionary lookup function in iOS 5) as well as word catalogs and word lists used for language related functions (spell checking, suggestions / autocorrect and hyphenation).