When you're writing in English and understand the language reasonable well I would say Artha is a first and best choice, compact but because based on Wordnet:
WordNet is a large lexical database of English, developed under the direction of George A. Miller . Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept. Synsets are interlinked by means of conceptual-semantic and lexical relations. The resulting network of meaningfully related words and concepts can be navigated with the browser.
It is of a very high quality; it is fantastic to get a more gutsy feeling for the meaning of many rare or more intricate words.
Are we more in translating I find Goldendict the most feature rich app.
Importing dictionaries is very easy.
Important also is that different kind of dictionaries can be imported; the interface is clean an efficient and you can import a lot of stuff -nearly everything that is in htm format- to make a encyclopaedia or so.
The special feature of Aarddict that is very useful for using an offline wikipedia file.
Stardict and Goldendict are online capable of online Wikipedia viewing.
All programs are through Aur available in Arch.