I’m not a lexicographer, but I know several from Twitter. That’s my disclaimer. What I’m writing here is taken from English-language dictionaries themselves (did you know the print versions usually include a “how to use this book” section?), personal experience, and Twitter discussions.
Dictionaries do not dictate how you are allowed to use a word. They do, however, tell you how words are used. Do you see the difference? They’re showing you a snapshot, in essence, of the English language at a moment in time. The definitions change with the language, but not as quickly as language changes. For a word to enter a dictionary, or for its definition to change, that word must appear in print in places where the lexicographers can cite it. That can be news media, fiction, nonfiction, periodicals, personal correspondence made public, transcripts of speech, websites, and so on.
Through these cites, the lexicographers are able to see how the word is being used at a specific moment in time. Their challenge, then, is to write definitions for words reflecting the current usages. If you want the inside track on how this happens, get your mitts on a copy of Kory Stamper’s Word by Word: The Secret Life of Dictionaries. (If you’re expecting it to be boring and dry, I hate to tell you you’re in for disappointment.)
You might also like to read about the furor caused when “ain’t” was included in the third edition of Webster’s Third New International Dictionary back in 1961. Have your smelling salts handy as you devour David Skinner’s The Story of Ain’t: America, Its Language, and the Most Controversial Dictionary Ever Published. Like Kory’s book, dry this one ain’t.
This article, also by Skinner, examines the history of the now-dispersed usage panel at American Heritage. Rather than take up space here trying to summarize, I’ll just point you at it. I will, however, say that even the usage panel didn’t provide be-all and end-all answers.
So what is a dictionary useful for, if it doesn’t dictate usage or meaning? What good are these snapshots?
First, it’s useful for finding the currently correct and accepted spelling(s) of words and phrases. Yes, even this changes. Compounds like “goodbye” often begin as two words (good bye), then become hyphenated (good-bye), and finally fuse, or close (goodbye). This process takes time, and a dictionary can tell you how a word is styled (with a space/open, hyphenated, or closed) at the moment.
Second, the dictionary will tell you when a word first entered English. This is helpful for writers of historical fiction; characters’ dialogue needs to be believable, and having someone use a word that hadn’t yet entered the language at the time the story is set is a sure-fire way to pull readers out of the book. One such word is “okay/OK.” If a story is set pre–Civil War (1861 to 1865), that word didn’t exist yet in the US. And it certainly didn’t exist in England or “on the Continent.” (Why, yes, I edit historical fiction. Why do you ask?)
Perhaps you’ve noticed I don’t refer to “the dictionary,” but to “a dictionary.” There is no such thing as THE dictionary. Merriam-Webster, American Heritage, Oxford, Collins, Macmillan are all major English-language dictionaries. The entries will be different. The spellings will vary. The definitions won’t match exactly. Oxford and Collins are British English. Macmillan is a blend, with lexicographers from both the US and Britain. And then there’s the Canadian Oxford.
I’m sure you have a dictionary in your home. (And on the off chance you don’t, head for your nearest library.) Open it. Read the front matter. Chances are, somewhere in that section are hints and tops about how the dictionary is organized, how the definitions are ranked, how spellings are ordered (the first one is the preferred, sure, but what about ones with “or” or “also” preceding them?). I’ll bet you learn something you had no idea about.