Pro- is a prefix that exists in two forms. The first comes from Latin pro, meaning “for.” In English, this pro- usually means “favoring” or “supporting,” as when it is prefixed to names of nationalities: pro-American. In this sense, the opposite of pro- would be anti: proslavery/antislavery. 1
The other pro- comes from Greek pro, meaning “before, in front.” The word prologue comes from Greek prologos, from pro- plus logos, meaning “speech.” In English, pro- often means “before” or “earlier” and is used mainly in science terms: prophase.

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