I think he/she probably means some dictionary software like Oxford, or Longman which could be locally installed in linux computer...
Of course, online dictionary is easier provided you have internet connections.
Some distributions like SUSE 10.0 are shipped with some dictionary software.

don't forget:
football - soccer

And all this applies to which distro?

Well, we're currently awaiting an explanation for the type of dictionary that uishen needs. So we're just chatting while we wait

You can try Artha, the open thesaurus written on Linux for English, which is light weight and has features like hot key lookup, regex based search, etc.

http://artha.sourceforge.net/

I don't think there are any dictionaries, however, as some other people suggested, there are lots of them online:
http://www.macmillandictionary.com/
http://dictionary.cambridge.org/
http://www.collinslanguage.com/
They are not as good as their commercial paper versions, though. I'm sure someone would have written a good dictionary for linux by now, if not for the fact that they would need all the definitions/examples which, I suspect, are copyrighted by bodies like Oxford, Cambridge, Macmillan, etc.

WordNet, the popular thesaurus work by Princeton University was developed completely on Linux; WordWeb in Windows are famous among win32 users, is based on WordNet. Artha also uses WordNet in Linux and has a large set of words and related words. I guess you haven't tried it, without doing so, you cannot deem that Linux doesn't have a dictionary written for it.

Is it a dictionary or a thesaurus? You seem to use those two words interchangeably. The two things mean something different. A dictionary entry contains (a) definition(s) of a word in question, as well as examples of the word used in context (meaningful sentences). A thesaurus, on the other hand, contains only synonyms/antonyms of the word in question (eg. big = large, huge, massive, etc.). Basically, they serve different purposes. For example, if you are writing some essay and don't want to repeat the word 'important' all the time, you would look it up in a thesaurus and learn that you could use words like 'essential', 'crucial', 'vital', etc. The main purpose of a dictionary is to provide you with the meaning(s) of a word and show you how it is used in context.

I recon the best english-english dictionary is in your browser (i.e. no need to install anything when there are so many online).

Here's a good educational definition on a suddenly frequently used word: http://en.wiktionary.org/wiki/bugger

Mons

Well, I did use dictionary and thesaurus interchangeably in this context; Reason: WordNet is a dictionary and a thesaurus. It gives you a word's definitions, examples on its usage and then all of its relatives like synonyms, antonyms, derivatives and a lots more on it. You can see Artha (a gui front end for wordnet)'s screenshots for a better understanding -> http://artha.sourceforge.net/wiki/index.php/Screenshots; also, it works completely off-line.

Looks interesting and it does have features of both. I just wonder why they refer to it as a thesaurus? If you look up the word 'thesaurus' in a dictionary (not in a thesaurus) you'll get something like:
According to wikipedia:

It's probably one of those 'misused' words: linux vs gnu/linux, hacker vs cracker, etc.

Artha? I thought she was Arthur's twin sister.

@sycamorex: I guess so too

@brianL: It is named after the Sanskrit word 'Artha'