What’s the difference between expedient and expeditious? The words were originally synonymous, but their meanings diverged long ago, and the distinction is small but significant.
Both words derive from the Latin term expedire, which means “disengage” or “prepare.” (The literal meaning is “free the feet.”) As you may have guessed, that’s also the source of expedition, in the original sense of “journey” and the associated meaning of “those on a journey.” (The adjective form is expeditionary.)
From expedire came the word expedite, meaning “dispatch or issue,” “do promptly,” or “speed up.” (Expediate is an unnecessary variant.) Two adjectives developed from this term: Expeditious originally meant “fitting” or “useful,” but it later acquired the sense of “promptly efficient” and lost its original meaning, which expedient retains.
Expedient has two connotations, once neutral and the other — more common — slightly pejorative: Something that is expedient may be suitable, but the word more likely reflects what is done out of self-interest or because it is most convenient rather than the best solution.
The adverbial form of expedient is expediently; the noun forms are expedience and, better, expediency. The adverbial form of expeditious is expeditiously, and the noun form is expeditiousness.
The unrelated word expeditation (the verb form is expeditate) is closer to the literal meaning of the Latin source; it means to declaw a dog or remove the pads of its feet to deter it from chasing animals — the word is therefore almost an antonym of expedite — and I hope it’s obsolete.