Just a guess...
It appears (from the message included in the screenshot below) that there is a
1. 'Spelling and grammar dictionary' targeted to preventing common spelling, spacing, and punctuation mistakes and then there is the
2. 'Oxford dictionary' for finding word definitions and context.
For speed, I'm guessing that the 'autocorrect spelling' function checks a few-thousand-word and perhaps a-few-hundred-rules lookup table, rather than the full OED.
So what happens if you 'learn' or 'ignore' the words marked by the spelling dictionary as incorrect?
Jeffrey Jones2 has given you the correct answer. Basically, the word "dictionary" led you to confuse two different functions -- spell-checking and reference.
Mac OS X provides a system-wide multi-lingual spell checker, which is available to any app which makes use of the Cocoa text engine. You configure the spell checker in System Preferences > Language & Text > Text > Spelling. The spell checker uses spelling dictionaries, but these are not dictionaries in the usual sense (a list of words with definitions or equivalents in another language). In their simplest form they are no more than lists of correctly spelled words.
Note that you are not limited to the spelling dictionaries provided by Mac OS X; you can add your own in <~/Library/Spelling> spelling. A little-heralded fact is that Mac OS X can use OpenOffice-format spelling dictionaries (.aff + .dic), which is very useful if you deal with lesser known languages.
Mac OS X also provides a reference function -- bundled with the system is the application Dictionary.app, which has nothing whatever to do with spelling. Rather, it allows you to access various reference sources, including dictionaries, a thesaurus, and -- of course -- Wikipedia; you configure them in Dictionary.app > Preferences. In addition to Apple's own dictionary, the US version of Mac OS X comes with the 2nd editions of the New Oxford American Dict and Oxford American Writer's Thes. Unlike Wikipedia, these are off-line dictionaries (stored on your machine, so you can access them at any time).
Again, you are not limited to Apple-supplied dictionaries. You can install others (if properly compiled) in <~/Library/Dictionaries>. Leopard simplified considerably the task of compiling Mac dictionaries, and now you can find in compatible format such well-known free dictionaries as BeoLingus (German) or CC-CEDICT (Chinese).
Dictionary.app provides to other apps the "Look Up in Dictionary" service. This service allows to select a word and automatically switch to Dictionary.app to look up that word in the selected dictionaries. Of course, it has nothing to do with spelling, hence the apparent discrepancy you noted.
In addition to Apple's own dictionary, the US version of Mac OS X comes with the 2nd editions of the New Oxford American Dict and Oxford American Writer's Thes.
All language versions of OS X have the same choice of dictionaries. In addition to those mentioned, for Lion they include Japanese, Japanese/English, Japanese Synonyms, the Oxford Dictionary of English, and the Oxford Thesaurus of English.
Well first of all thanks I meant my earlier reply for you but I am visually impaired an tend to get things out of order.
I do have a question where are the spelling dictionaries?
When I look into the spelling folder in my Library there is one but it has only one word in it. When I look into the spelling folder in the system wide Library it is empty.
It appears to me that the spelling dictionaries are binary files inside the AppleSpell service in the System Library:
I hope it goes with out saying that you should not tamper with these.
Any new words that you tell the spell checker to "learn" are in your user Library.
Even if the built-in spelling dictionaries were editable, it would not be a good idea to add to them your own modifications, because they could be overwritten by a system update. Much better to follow the Unix/Mac OS X paradigm of separate system, local, user domains.
You mentioned 2600 words. If you need to spell check specialised jargon (eg, medicine), try looking for OpenOffice-compatible spelling dicts (eg, OpenMedSpel for medicine). Once you get an OpenOffice extension file (.oxt), just change its file name extension to .zip, decompress it, and copy the .aff and .dic files to <~/Library/Spelling>. (No need to bother with any other files in the archive.)
For (free) off-line reference and translation, here are a few suggestions:
Pour nos amis francophones, Le Dictionnaire de l'Académie Française, 8ème édition (1935).
(Yes, it is a bit old, but some things, just like a good French wine, get only better with age. The 9th edition, a work in progress, is available on-line. Larousse and Le Robert are partly available on-line; the off-line versions are not free and are not compatible with Dictionary.app.)
If you're learning German, you have your choice of BeoLingus
If you're learning Chinese, both Píng guǒ
and the Xiǎo cí diǎn set
are based CC-CEDICT.
Finally, as a piece of minor trivia, if you install the Server Admin Tools, you also get Apple's Information Technologies Dict (but which, unfortunately, is neither comprehensive, nor up-to-date).