What’s the difference between extraneous and superfluous? Are they redundant to each other, and how do these terms relate to redundant and other synonyms?
Extraneous, which stems from the Latin term extraneus (related to strange), means “irrelevant” or “nonessential, or “coming from or existing outside.” Extravagant means “beyond what is reasonable or appropriate,” with multiple corollary senses having to do with such qualities as cost or decoration; its second element is related to vagary and vagrant and means “wandering,” so the literal translation is “going beyond.” Extrinsic (from a Latin word meaning “from without”) is a direct synonym of extraneous.
Superfluous (from Latin, and literally meaning “overflowing” — the second part of the compound is related to fluid) means “extra, more than is necessary.” Supererogatory is identical in meaning to superfluous, with an additional sense of “done beyond what is required.” Supernumerary means “exceeding the usual,” “more numerous,” or “not what is usually included”; it is also used in noun form to refer to an extra person, especially in theater as a more formal equivalent of the film-production slang extra. The second element is from the same Latin term from which numeral and number are derived.
Redundant has the same literal meaning as superfluous — the second part of the compound is related to wave and is seen, slightly altered, in the word inundation — and the identical basic connotation, though it also has the senses of repetition, abundance, or extravagance, or duplication as a safety measure. More quotidian synonyms include excess, spare, and surplus.