Prefixes are key morphemes in English vocabulary that begin words. The English prefix ob- usually means “against, in the way” or “towards.” An example of ob- when it means “towards” is the word obligation, or a tying of some duty “towards” you; in the case of ob- meaning “against” or “in the way,” the word obstacle" refers to that which stands “against” you, “in the way” of proceeding from point A to point B.
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