Please, oh, please, consider adding a thesaurus. There is no more frequent tool that a writer uses and a good one is worth its weight in gold. 
Why add a thesaurus when OS X has one built in?
… and the free NIsus Thesaurus, and the free CleverKeys service.
Right click on a word in Scrivener (or control + left click) brings up a menu that accesses dictionary/thesaurus. If you go into the dictionary preferences, you can tell it to list the thesaurus entries first.
“Any word you have to hunt for in a thesaurus is the wrong word. There are no exceptions to this rule.”
– Stephen King, writer
cu!
jonas
Writer’s can be arrogant, can’t they? The only exception is if you are NOT Stephen King.
This has nothing to do with arrogance. I think what King means is: just trust your own words.
You may be right - but trusting Stephen King’s vocabulary is a whole universe different than a less-experienced writer trusting his/her words, don’t you think? A thesaurus helps people think about the nuances of words.
I just take exception with writers telling other writers how to write. Writing can not be taught, it can only be learned. You have to find your own way, otherwise you’re just writing like someone else.
A sentiment with which I think King would agree. His book on writing is one of the few that’s worth anything, IMO.
In the martial arts and eastern meditation disciplines, they say that a teacher’s function is to light the way for those who come after. I think that’s true in writing as well. You have to find your own way, but having a good teacher can save you a lot of stumbling around in the dark.
Katherine
I agree, the King book is good.
Another excellent book on writing is Bone by Bone, by Anne Lamott - really about the psychological bagagge we writers carry around - reading it can be quite liberating.
Speaking of psychological baggage. 
One Application I love is Grammarian Pro X
linguisoft.com/gramprox.html
What is nice is it works in ALL OSX applications and is a bit more intuetive than the built in OSX dict/thes. Has a few more options and is really a nice app for writers. It also offers statistics for word count, sentence count, average syllables per word, passive sentence percentage, and applies the Flesch, Flesch-Kincaid, and Gunning FOG tests for average readabitlity, etc. Auto Correction, Text cleanup, and adjustable writing styles for Dict/Thes lookup and grammar correction.
You can apply the statistics to selections (such as a highlighted paragraph) or the whole document.
Real nice app IMO
With my mushy brain just now, my vocabulary rarely extends much beyond about 200 words at any one time,
(and most of those are to do with the bodily functions of small children). 
Obviously this is not much use for writing for anyone over the age of about four, so I find a thesaurus invaluable.
I use: http://thesaurus.reference.com/
It’s not perfect, sure, but it helps. Sometimes the right word just pops into my head anyway, while I’m looking at a list of synonyms.
I’m a big fan of extending one’s vocabularly, and the use of a thesaurus (preferably in conjunction with a dictionary) is a great opportunity to achieve that aim.
The trick is not to overdo it.
Regards, Leigh
Because there are problems and limitations to that dictionary. It’s fine if you want to look up the meaning of a word, but pretty useless as a thesaurus. For all too many words, there are no synonyms offered; those words that do have synonyms listed don’t have many of them, and they’re not well or clearly listed.
This is a wish list; that being the case, my wish is NOT for OS X’s very limited dictionary/thesaurus, but for one much more like MS Word’s. One that I can have as a button in my tool bar, one that offers me a window that stays up and is always there if I need it (that I don’t have to re-open with every new word), one that offers seperate windows of definition and synonyms–and lists those synonyms very neatly and clearly. One that allows me to click on a synonym and get THAT word’s meaning to make SURE that’s the word I want.
For example, if I pick the word “Sensitive” the MS Word thesarus offers me definitions in one window, as well as single-word “meanings” another window: responsive, thin-skinned, precise, and a list of synonyms in a third window. Which meaning of sensitive do I mean? I click on any one of those meanings and get a new list of synonyms. Excellent! I can even click on the synonym of my choice and REPLACE the word in my document. Double excellent.
OS X dictionary, on the other hand, gives me a bunch of definitions for “Sensitive,” and NO synonyms. And that’s all it offers. No way to click on those definitions and get synonyms that relate to them, no way to replace words…I don’t know about you, but this is not MY ideal writer’s thesaurus.
So, in answer to your question THIS is why I want a dictionary/thesaurus for Scrivener even though OS X has one. This is why I want one in spite of the fact that there are plenty of excellent ones on the internet. Because while I’m gripped by the muse and writing away, I want the quickest way to find the right word. Option-click to OPEN the dictionary and finding NO synonyms for that word is going to interrupt my writing, not facilitate it.
Agreed. Rules like this push all my buttons. In my experience, Writers who say there are “no exceptions” really mean is there are no exceptions “If you want to write like me!” or, more to the point, “If you’re me.” And I’ve no desire to be Stephen King.
I, myself, have only one “no exception” rule: Do whatever is needed to make the story the best it can be. If using a thesaurus helps me do that, then I’m using a thesaurus. As for Stephen King’s advice in regards to that, he’s not writing the story, he doesn’t know what it needs or why it needs it; his name isn’t going to be on it, and he’s not going to suffer the consequences of it turns out badly because I did what he said. And neither is the person who, all unasked for, quoted this advice as if I needed or wanted it.
I think that pretty much explains how I feel about being preached to on the subject of writing.
I do not really know if this is possible or not, but if making a dictionary/thesaurus is to much of a pain. Perhaps integrating a already existing one would be easier. Example being, having a shortcut key or icon that activates whatever 3rd party dictionary/thesaurus program you do have on your Mac? Just throwing ideas because a good dictionary/thesaurus is very important.
See michpen’s note, above. Mac OS X has a built-in dictionary and thesaurus, available in ALL programs (except Microsoft). Just control-click on any word.
My experience: Dictionary must be in the Applications folder, but not inside a sub-folder within Applications. An odd hangover from earlier versions of the OS, which did not allow sub-folders.
I sympathize, but adding this tool would bloat Scrivener. It’s for outlining and drafting. Anything else one might need (encyclopedia, bibliography, timeline, mindmap, etc.) exist in other applications or on the Internet.
Here are some of my favorite word sources, all free and online:
Online Etymology Dictionary: etymonline.com/
OneLook: onelook.com/
WordIQ: wordiq.com/
WordNet: wordnet.princeton.edu/
RhymeZone: rhymezone.com/
Here’s a vote for no attached or incorporated reference texts. They’re easily enough accessed within Mac.
When I’m gripped by the muse – as warmly now as when I was young, but somewhat less often – I continue the dance for as long as she’ll stay. Asking her to wait a moment while I flip through a thesaurus is worse than impolite; it’s impolitic. She’s not a fickle creature, but she is temperamental, apt to vanish in a moment if my attention flags.
Reference texts are for when she’s gone, when time can comfortably be given over to technical chores – excising adverbs, pruning prepositions, tracking down sharper verbs and more precise nouns.
Just to reiterate that Scrivener is not going to have its own built-in dictionary or thesaurus. The one built into OS X is already good enough, and you can already double-click on a word, ctrl-click and select “Look Up in Dictionary”. From there you can click on “Thesaurus” or scroll down and look for alternative words.
As a one-man show there is simply no way that I’m going to put any development time into creating my own dictionary or thesaurus - that way madness lies!
All the best,
Keith